Rivet recovery method

ABSTRACT

A method of recovering rivets from a rivet hole prior to the deformation of the rivets by the rivet dies includes the steps of positioning a suction cavity over the rivet in the rivet hole, pushing the rivet out of the hole with a rivet tail die and blocking the hole with the rivet tail die to prevent suction from the tail-side pressure nose from holding the rivet in the rivet hole. The swirling air in the suction cavity induced by the suction hose connected to the suction cavity pulls the rivet out of the hole and sucks it out of the suction cavity through the suction hose.

BACKGROUND OF THE INVENTION

This is a division of U.S. application Ser. No. 07/632,445 filed on Dec.21, 1990 and entitled "Drill/Rivet Machine", now pending.

This invention relates to a machine and its constituent components fordrilling a hole in a workpiece, inserting a rivet in the hole, andupsetting the rivet, all without moving the lateral position of themachine or the workpiece, and to an assembly for supporting andpositioning a plurality of said machines for working simultaneously onthe workpiece.

Riveting machines are well known and in wide use throughout theaerospace industry as well as other industries. Rivets provide the bestknown technique for fastening an aerodynamic skin to a frame to providea strong aerodynamically smooth surface. Rivets are also used in theinterior structure of the aircraft since they are the lightest and leastexpensive method of fastening structural components together. However,inserting and upsetting rivets is a labor intensive process and for themost part is done manually, one rivet at a time. The work is extremelytedious and yet at the same time requires a highly skilled operator toproduce quality rivets consistently, and highly skilled quality controlinspectors to insure that all of the rivets meet the specifications offlushness, interference and button formation. These personnel costssubstantially increase the cost of a riveting operation and tend tooffset the inherent low cost of the rivet itself. The process appearssusceptible to automation, and attempts have been made for many years toobtain the benefits of increased capacity and quality while reducingcost and rework, but attempts to develop automated rivetinginstallations have been hampered by a multitude of practical problemswhich interfere with the smooth operation of an automated system,resulting in a requirement for continual manual intervention by skilledoperators.

A riveting operation is often so noisy that, in a factory where largescale riveting is being done, hearing protection is mandatory. Mucheffort has gone into quieting the riveting process to protect employeesfrom hearing damage, but the work place remains noisy and hearingprotection remains mandatory. Hydraulic rivet squeezers are quiet, butthey require massive mounting structures to withstand the reactionforces exerted by the hydraulic squeezer. The cost and size of thesemounting structures prohibit their use in many applications.

The force to upset a rivet is typically on the order of five to fiftythousand pounds. This force must be exerted from both sides of therivet, either by a reaction force through a large C-frame, a reactionmass, or an active force generator on both sides of the rivet. Hand-heldpneumatic riveters are one simple solution but require two skilledoperators, are not always repeatable, are noisy, and have been linkedwith carpel tunnel syndrome. The ideal riveter would be anelectromagnetic, one or two blow riveter acting virtually simultaneouslyon both sides of the rivet.

A typical automated drill/rivet machine has a drill which will drill ahole into the workpiece and then shift to one side so that the rivet maybe inserted. The rivet insertion mechanism then shifts to one side sothe rivet may be upset by a rivet die acted on by a driver. The shiftingof the drilling and insertion mechanisms is time consuming and requiresextremely precise positioning mechanisms in order to maintain thenecessary alignment of the drill, the rivet inserter, and the rivet dieand driver. A preferable technique would be to perform all threefunctions from the same lateral position of the device so that noindexing of the major components is required. Some lateral movement willstill be necessary to feed the rivet into the hole and position the dieover the rivet so that the rivet can be upset. The moving structure forthis lateral shifting should be made as light as possible so that it canbe indexed from position to position quickly and with great precision.

The drill chips that are produced by the drilling operation arepotentially troublesome because they can interfere with movement of thedie shuttle mechanism and, if they get between the workpiece and thepressure foot which clamps the workpiece in place, the pressure footwill embed the chips into the workpiece and mar it and could affectdrill or countersink depth. The conventional technique for disposing ofchips is to provide a nozzle to blow the chips away from the work siteor a suction hose to suck the chips off of the work piece after theyhave fallen onto it. These techniques reduce the problem by removing themajority of the drill chips, however there are always some chips thatare not removed and these can cause problems and must be periodicallyremoved from the work site by an operator with an air hose. A drillrivet machine would be much more reliable and produce a product withmuch fewer defects if a reliable chip removing system were available toremove all of the drill chips before they can even come into contactwith the workpiece so that the workpiece and the work site are keptclean and free of drill chips.

The drill in the drilling operation must be periodically lubricated topreserve the life of the drill, to maintain hole quality, and to speedthe drilling operation. However, in many applications the lubricant isconsidered a contaminant to the workpiece because it adversely affectsthe coatings on the workpiece or because of subsequent operations whichmust be free of lubricant on the surface. The best available techniquefor lubricating the drill without contaminating the workpiece is alubricant spray mist system which blows a mist onto the drill andlubricates only the drill and not the workpiece. However, the mist tendsto settle to some extent on the workpiece and therefore the mistlubrication system is not one hundred percent effective in maintainingthe cleanliness of the workpiece. A lubrication system for a high speeddrill rivet device should lubricate the drill before every drillingoperation and should reliably protect the workpiece from contaminationby the lubricant.

A drill rivet device must accomplish several operations and do soquickly and precisely. It would be desirable during drilling, rivetinserting, and rivet upsetting that the work- piece be clamped securelyin a single unmoving position so that the axis of the machine remainsaligned with the position at which the rivet is to be placed, despitereactive flexing of the holding fixture. The drill and the rivetupsetting device should be mounted for precise axial movement while thework- piece is held in a clamped position, but then must be mounted sothat it can be retracted and the workpiece unclamped after the rivet isplaced to enable the workpiece or the machine to be moved to the nextposition where the next rivet is to be placed. Ideally, all of thesefunctions should be accomplished in a single small, light weightmechanism that would make it possible for the drill rivet machine tooccupy only a small volume so that it does not interfere with otheradjacent mechanisms.

Within the frame which guides the drill and the rivet upsettingmechanism, the axial movement of the drill and the riveter must beguided, cushioned, damped and positioned so that it accomplishes thefunctions for which it is intended at high speed and low impact. In anelectromagnetic riveter, the impact created when the coil is energizedshould be absorbed in a recoil cushioning system which enables themachine to take advantage of the possibilities for low reaction force inthe electromagnetic riveter. However, the translations involved must beheld to a minimum to insure that no wasted motion occurs so the machinecan operate at its highest possible cycle rate.

One of the factors that has delayed the development the development ofautomated riveting operations in the past has been the large mass of theequipment that must be moved and precisely indexed to the location atwhich a rivet is to be placed. Improvements have been made but thelateral movement of mechanisms remains a problem and causes the machinecycle time to be lower than it potentially could be. Exacerbating theproblem is the multitude of functions which must be performed and thesubstantial precision with which these functions must occur in precisealignment over the rivet placement location. This precision oftendegrades as the machine ages and the guideways for the shuttlemechanisms wear.

One of the functions in an automatic riveting machine that must operatereliably is the rivet feeding and insertion device. In someapplications, the rivet is fed to the machine from a rivet blow feederthrough a rivet feed tube, in which the rivet can attain substantialvelocity in order to sustain fast machine cycle times. If the rivet wereto impact the workpiece at its maximum velocity in the feed tube, itcould damage the work- piece in the marginal regions around the hole inwhich the rivet is to be inserted and could also damage the rivetitself. These dents and nicks in the rivet and the workpiece regionsaround the hole can influence the sealing of the rivet in the hole andalso potentially prevent the rivet from entering the hole at all. Thusit is necessary to insure in an automated riveting operation that therivet approaches the workpiece at a velocity that is fast enough tocarry the rivet into the hole but not fast enough to damage the rivet orthe workpiece.

Rivet attitude as it approaches the hole should be controlled so that itenters the hole without jamming against the edges of the hole orotherwise jamming in the feeding operation. In conventional rivetfeeding systems, the rivet is held in a rivet gripper and is inserted inthe hole. This system works well most of the time, although occasionallythe rivet gets cocked in the hole and the rivet dies smash it in thecocked position, creating a difficult repair job. However, it doesrequire a finite time for the rivet gripper to place the rivet in thehole, and is just one more thing to wear out of tolerance. A preferablesystem would have no moving parts for rivet insertion, and would bevirtually instantaneous in the insertion of the rivet in the hole, forminimum machine cycle time.

The rivet, once seated in the hole, must remain in position until it isupset. Some rivet operations are performed in an upside-down orientation(that is, with the headed end of the rivet facing downward) and if therivet is not held in position it could fall out before it is upset, orit could slide out of position and jam the feed mechanism or the rivetdie shuttle. The holding system must function from the time the rivet isinserted until the time the rivet is upset so that the rivet ispositively held in the correct position at all times.

The rivet feeding tube usually enters the machine from the side becausethe rivet die driver and drill are in an axial position and couldinterfere with the path of the rivet coming axially into the hole. Thusthe rivet enters laterally into the machine and then follows a curvedpath to straightened its line of travel and align it with the hole,which is on the machine axis. The change in direction of the rivet is atricky operation because several different types and sizes of rivets maybe fed in a automatic riveting operation and although the rivet can besofter material such as aluminum it still can cause considerable wear ina curved feed path. Moreover, the tighter the bend that the rivet mustexecute in going from the lateral approach to the axial path of travelto enter the rivet hole, the more likely it is that the rivet will jamin the bend.

When the rivet exits the bend, it is in an unstable condition and mustbe straightened and stabilized so that when the rivet leaves the end ofthe feed structure and moves into the hole, it will travel in a stablecondition and does not become canted, so that it enters the holestraight without becoming jammed diagonally in the hole.

A rivet is precisely sized for the thickness of the workpiece which itis to hold together and the stress which it is to carry. The impactenergy of the rivet driver is designed to completely form the button endon the rivet and cause the desired degree of interference between therivet shank and the hole, and between the rivet head and the surface ofthe workpiece, in the case of Briles rivets. Any substantial deviationfrom the design parameters will result in an improperly formed rivet ora damaged workpiece. For example, a rivet which is too long will havemore material to be strained than the driver has energy to strain andtherefore the rivet will be incompletely upset, resulting in ainsufficiently formed button on the end of the rivet and inadequatedegree of interference between the shank and the hole, and between thehead and the surface of the workpiece. Similarly, a rivet which is tooshort will have an insufficient amount of rivet material to absorb thedriver impact and therefore the rivet tail will be flattened and theenergy in the driver will have to be absorbed by the workpiece,resulting in a deformed or a "dimpled" workpiece, or a rivet head pushedoff of its countersink. These defects require rework which is expensiveand slows the production output of the plant. It would be desirable todesign a system for measuring the length of the rivet before it is upsetto insure that these defects do not occur.

In the event that a rivet is detected that is too long or too shortthere should be some method for removing the missized rivet from thehole and disposing of it so that a properly sized rivet can be inserted.This operation should be done quickly and reliably so that theefficiency to be gained by an automatic riveting operation is not lostby these recovery operations. Ideally, the rivet measuring technique andthe rivet removing and disposing technique can be incorporated inmechanisms which are part of the automatic drill rivet device withoutadding undue complication or increased cost to the device.

An electromagnetic actuator for an electromagnetic riveter has a highamperage coil which develops considerable heat from electricalresistance over a period of use. The heat raises the electricalresistance of the coil and therefore the voltage must be raised, therebyreducing the efficiency of the operation. More importantly however thetemperature of the coil must be held within certain limits to preventthe electrical insulation and other materials from reaching breakdowntemperatures. The conventional methods for coil cooling employ anairflow through and around the coil to extract the heat and carry itaway in the air stream. However, the direct cooling of the coil in thismanner requires that the air be dry and of high purity. Water vapor inthe air striking the hot coil can cause corrosion of the coil, andimpurities in the air can collect in and around the coil to causebridging of the coil windings. The air treatment system is reliable andavailable, but it is expensive, requires periodic maintenance and isbulky. An improved coil cooling method for an electromagnetic actuator,that does not require the expensive drying and filtering equipment,would be a great benefit to the operators of electromagnetic actuatorsin high power equipment such as an electromagnetic riveting machine.

The rivet forming technique for slug rivets includes striking the riveton both ends simultaneously so that the rivet shank is deformed toprovide shank/hole interference, and the rivet head is deformed toprovide a properly formed button at each end of the rivet. This providesthe optimum rivet strength. Headed rivets present a slightly morecomplicated problem. Headed rivets already have a head formed at one endof the rivet, normally tapered so that it fits into a countersunk holein the surface of the workpiece. In order to provide the proper holdingeffect, the rivet must be installed in such a way as to insure that thehead will engage the work- piece countersink with sufficientinterference or pressure when the riveting operation is completed. Withthe old fashioned pneumatic riveting machines, which deliver multipleblows against the rivet, it was possible to determine by simpleinspection when the rivet had been sufficiently deformed because theincremental amount of deformation created in the rivet by each blow isso small that a skilled operator could inspect the rivet and see thatthe operation was complete for that rivet. Additionally, he could tellby the sound and feel of the machine when the rivet was deformedsufficiently to create an acceptable rivet. However, the situation forelectromagnetic riveters is more complicated because the entire rivetupset operation must be accomplished in one or two blows, so the blowsmust be timed and powered to accomplish the entire rivet upset operationcompletely. The prior art has always delivered the identical blow toboth sides of the rivet because of the apparently logical assumptionthat identical blows will prevent asymmetric effects on the rivet andprevent damage to the workpiece. However, we have discovered that thefunctions to be performed at each end of the rivet are considerablydifferent from each other and therefore the blow to be delivered to therivet should be tailored to the function which it is to perform. Sincethese functions are different for different kinds of rivets, theparameters of the blow to be delivered need to be adjusted in each case.

A riveting machine which drills holes in a workpiece, inserts a rivetand upsets the rivet, all automatically and at high speed, should beunder precise automatic control and provide feedback to the controllerthat every operation has been completed before the next operation can bestarted so that the danger of jamming or damaging the machine byclashing of subcomponents is minimized. This implies that the machine beprovided with sensors which indicate when a operation has been completedand with automated control rules which cause the machine toautomatically halt when a failure has occurred to prevent massive damageto the machine or workpiece. The suite of sensors should be as simpleand reliable as possible to minimize the cost and maintenancerequirements, and should provide all the sensing operations necessary togenerate the feedback data needed for the machine to function reliably.Finally, when a failure occurs, the sensor and control system shouldprovide information to the operator or the maintenance personnel as tothe cause of the failure was so that corrective action can be taken toidentify the problem, clear the blockage or make any repairs that arenecessary.

The bearing support at the lower end of the drill and the EMR driver ina concentric arrangement is a desirable function because without it thedrill tends to wander and produce a misplaced or misshapen hole, andbecause the driver should produce its force pulse at exactly the rightlocation to minimize the chances of damaging the machine or theworkpiece. The prior art techniques for accomplishing these functionsinclude special drills with bearing surfaces, but that is an expensiveapproach that requires the stocking of special parts whoseunavailability would make the machine unusable. Moreover, they alsotypically require the stacking of tolerances and clearances which areadditive and can result in excessive play at the free end of the drillor the driver.

The coil of the EMR is electrically insulated from, but in mechanicalcontact with a transducer of high electrical conductivity. Theconvention arrangement is to put the coil in contact with the transducerand the transducer in contact with the driver, so that when a burst ofcurrent is sent through the coil it will create a rapidly increasingmagnetic field which will induce a current in the transducer, which inturn generates an opposing magnetic field and creates a strong repulsiveforce between the transducer and the coil. The transducer forces thedriver against the rivet die to upset the rivet. Arrangements of thistype have worked well for numerous years but suffer from the placementof the heavy cable that carries the large amperage current to the coil.Since the coil recoils away from the transducer and the driver alongwith the recoil mass, a substantial translation of the coil occurs,which means that cable must be capable of withstanding this severewhipping motion. The cable thus must be reinforced and strengthened sothat it does not suffer damage from fatigue effects of these continualsharp translations. It would be desirable if the coil could be mountedin such a manner that it experiences a very small translation so thatthe extra reinforcement in strengthening of the cable would beunnecessary.

Maintenance of an electromagnetic riveter, particularly when it isincorporated in a concentric drill/rivet machine, can requiresubstantial nonproductive time during which the machine is beingdisassembled, serviced, and then reassembled. It would be of greatpractical value if the disassembly time could be reduced and the machinedesigned in modules that could be replaced with new modules when worn,so that the nonproductive time could be reduced or eliminatedaltogether.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improveddrill/rivet machine with a concentric drill and driver that can operatereliably with high precision and at a high cycle rate to obtain improvedproduction capacity and reduced cost.

It is another object of this invention to provide a chip suctionapparatus for preventing contact of the drill chips with the workpiece,and for removing all of the drill chips so the workpiece and equipmentare maintained substantially free of drill chips to prevent interferenceby the chips with the operation of the device, and to prevent embeddingof the chips in the workpiece by the pressure foot.

It is still another object of this invention to provide an improvedvacuum mist lubrication system for a drill, for blowing mist lubricationon the drill and maintaining the workpiece free of lubricant.

It is a further object of this invention to provide a frame for adrill/rivet machine which allows the workpiece to be securely clamped inthe required position while the drill, shuttle and riveter can movefreely and precisely to their required positions.

It is yet a further object of the invention is to provide an improvedpneumatic recoil cushioning, damping and positioning system that enablesthe machine to work smoothly, quickly and quietly by holding thetranslations to the shortest possible and minimizing the recoil forcesso that the mechanism can be small, lightweight, nimble and inexpensive.

It is still a further object of this invention to provide an improvedrivet insertion device that enables insertion of the rivet into the holedrilled by the machine in a manner that does not cause damage to therivet nose or to the marginal portions of the workpiece around the hole,and which holds the rivet in the hole in all positions until the riveterupsets the rivet.

A still further object of the invention is to provide an improved methodof detecting rivets which are too long or too short before the rivet isupset, and for removing and discarding the outsized rivets so that acorrectly sized rivet can be inserted.

A Yet further object of the invention is to provide an improved methodof cooling an electromagnetic actuator coil which does not requirespecially filtered or dried air so that ordinary filtered room air canbe used and the conventional expensive air treatment system isunnecessary.

A still yet further object of the invention is to provide electricaladjustments to the electromagnetic actuator circuit to adjust the forcepulse acting on the two ends of the rivet so that the correct pulse isdelivered to each end of the rivet to optimally perform the rivetingfunction at that end.

Still a further object of the invention is to provide an improved sensorsuite and a method of operating the drill/rivet machine using feedbackfrom the sensors to insure that the multiple operations of thedrill/rivet device can be performed in rapid sequence without damagingthe machine or the workpiece in the case of a failure or a jam in theoperation.

Yet another still further object of the invention is to provide animproved bearing system for supporting the lower end of the rivet driverand the drill to insure axial alignment and close toleranceconcentricity of the drill and the driver.

Another object of the invention is to provide an improvedelectromagnetic riveter having a coil which experiences a very shorttranslation so that the cable to the coil can be attached to astationary mount and does not need expensive and bulky specialreinforcement to enable it to withstand whipping motions caused by longcoil translations of high velocity.

Still another object of the invention is to provide an improveddrill/rivet machine that is configured for fast and easy disassembly sothat maintenance can be done on the machine with minimal or no time inwhich the machine is taken out of productive service.

These and other objects of the invention are attained in the preferredembodiments disclosed herein of an assembly having ganged concentricdrill/rivet machines disposed on a frame which can move longitudinallywith respect to the workpiece, itself held on a movable frame, andwherein the electromagnetic riveters can be moved along the frame sothat the spacing between the riveters can be varied. Each of theseriveters includes an electromagnetic actuator which is cooled by contactwith a finned transducer so that heat is transferred from the coil tothe transducer. The transducer fins efficiently transfer heat to anairflow blowing around the transducer and across the fins to provide theheat extraction path for the coil and the transducer. The power supplyfor the electro- magnetic actuator coil has a time delay so that theriveter on the tail side of the rivet lifts the head of the rivet out ofthe countersink before the headed side of the rivet is struck, to insurethat the force of the riveter is delivered to the rivet and is notabsorbed by the workpiece, so the rivet shank is upset to provide properinterference with the hole in the workpiece. Alternatively oradditionally, the riveter power supplies have different capacitances sothat the current pulse to the electromagnetic actuators powering thedrivers on the two sides of the rivet can be adjusted to balance theforce on both sides of the rivet despite differences in driver stroke,driver work requirements, driver momentum, and coil/transducerseparation, all of which affect the requirements for the force pulseexerted by the driver on the rivet, and through the rivet on the panel,so that sufficient force is delivered to properly deform the rivetwithout dimpling the workpiece.

DESCRIPTION OF THE DRAWINGS

The invention and its many attendant objects and advantages will becomemore clear upon reading the following description of the preferredembodiment in conjunction with the following drawings, wherein:

FIG. 1 is an isometric view of a positioner for holding and positioninglarge aircraft body panels for drilling and riveting by drill/rivetmachines;

FIG. 2 is an isometric view of a motive apparatus for moving andorienting the end effectors on the carrier of the positioner shown inFIG. 1;

FIG. 3 is a schematic view of a motive mechanism for moving the positionof the carrier shown in FIG. 1;

FIG. 4 is an alternative motive system for moving and orienting the endeffectors shown in FIG. 1;

FIG. 5 is a schematic diagram of a control system for controlling thepositioner shown in FIG. 1;

FIG. 6 is an isometric view of one of the pairs of drill/rivet machineswhich are mounted on the end effectors shown in FIG. 1;

FIG. 7 is an exploded isometric view of a cage on which the componentsof the drill/rivet device are mounted;

FIG. 8 is a plan view of the middle plate of the cage shown in FIG. 7,showing the drill motor and lead screw assembly attached;

FIG. 9 is a plan view of the top plate of the cage shown in FIG. 7;

FIG. 10 is a sectional elevation of a portion of the right hand unitalong lines 10--10 in FIG. 6, showing the ball screw for plunging thedrill;

FIG. 11 is a sectional elevation of a portion of the right hand unitalong lines 11--11 in FIG. 6, showing the drill motor, the cagecylinders, and half of the electromagnetic riveter;

FIG. 12 is a sectional elevation of the electromagnetic riveter mountedin the cage of the drill/rivet machine;

FIG. 13 is an enlarged sectional elevation of the electromagneticriveter with its recoil, damping and pop-back cylinders;

FIG. 14 is an exploded perspective view of the support member andbearing for the drill quill and the driver of the electromagneticriveter;

FIG. 15 is an exploded perspective view of the shuttle shown in FIG. 6for moving the drill cavity, rivet inserter and rivet die stations inorder under the axial centerline of the machine;

FIG. 16 is a plan view of the shuttle shown in FIG. 15, and showing thedrill cavity aligned with the axis of the machine;

FIG. 17 is a plan view of the shuttle shown in FIG. 15 with the rivetinsert station aligned with the axis of the machine;

FIG. 18 is a plan view of the shuttle shown in FIG. 15 with the diecavity aligned with the axis of the machine;

FIG. 19 is a perspective view of the rivet guide shoe in the shuttle,showing the rivet guide chute offset from the rivet brake and stagingtube in the shuttle, with a rivet held in a river staging tube inpreparation for feeding into the rivet guide chute;

FIG. 20 is a figure similar to FIG. 19, but with the rivet staging tubealigned with the rivet guide chute and showing the rivet having passedthrough the tube and the chute, and jumping the gap toward theworkpiece;

FIG. 21 is an elevation, partly in section, of the rivet guide tubeshown in FIG. 15, and illustrating a method of manufacturing the taperedlip;

FIG. 22 is a sectional elevation of the shuttle showing the rivet insertstation of the shuttle aligned with the machine axis;

FIG. 23 is a sectional elevation along lines 23--23 in FIG. 22;

FIG. 24 is a sectional elevation of the shuttle showing the drill cavityof the shuttle aligned with the machine axis, and the drill justfinished drilling a hole in the workpiece;

FIG. 25 is a sectional elevation of the shuttle after retraction of thedrill and shifting of the shuttle body to align the rivet insert stationwith the machine axis, and showing the line of travel of a rivet intothe hole drilled in the workpiece;

FIG. 26 is a sectional elevation of the shuttle showing the shuttlemoving from the rivet insert position to the rivet die position andshowing the path of airflow into the pressure foot to hold the rivet inplace during movement of the shuttle;

FIG. 27 is a sectional elevation of the shuttle showing the rivet diestation aligned with a machine axis and showing a portion of driveracting on the rivet die to upset the rivet;

FIG. 28 is a sectional elevation along lines 28--28 in FIG. 16

FIG. 29 is a partial sectional elevation along lines 29--29 in FIG. 18;

FIG. 30 is a schematic diagram of the pneumatic system for controllingthe motive functions of the machine;

FIG. 31 is a schematic drawing of the sensor suite for the drill/rivetmachine shown the in FIG. 6.

FIGS. 32A-F are sections of a logic diagram and operation sequence chartfor the drill/rivet machine of this invention;

FIGS. 33A-F is a series of schematic diagrams showing the rivet recoverysequence;

FIG. 34 is a schematic diagram of the end effector controller;

FIG. 35 is a schematic diagram of the control system for theelectromagnetic actuator in the drill/rivet machine of this invention;

FIG. 36 is a graph showing the time distribution of current from the twocapacitors through the coils of the electromagnetic actuators;

FIG. 37 is a graph showing the force exerted on the drivers and on thebuck side of panel by the rivet when the capacitance and voltage areequal in the head and tail riveters;

FIG. 38 is a graph showing the force exerted on the drivers and on thebuck side of the panel by the rivet when the capacitance and the voltageare uneven in the head and tail riveters; and

FIG. 39 is a sectional elevation of an alternative embodiment of theelectromagnetic riveter of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, wherein like reference characters designateidentical or corresponding parts, and more particularly to FIG. 1thereof, an airplane body panel fabrication positioner 30 is shownhaving a positioner frame 32 on which an aircraft body panel 34 ismounted for riveting, for example, of body stringers to an aluminum bodyskin. A plurality of locating ears 35 project from the edges of the bodypanel 34 and a key indexing hole 33 is drilled in each ear for use inprecisely locating the panel on the frame. The body panel will bereferred to generically hereinafter as a workpiece and could be a bodypanel, a wing panel, or numerous other parts of the airplane thatrequire riveting operations.

A carrier 36 is shown mounted on a machine base 37 for movementlongitudinally therealong relative to the frame 32. The carrier 36 has acentral opening 38 through which the frame 32 and the workpiece 34extend. A number of pairs of end effectors 40 are mounted on the carrier36 on both sides of the opening 38, facing the workpiece 34. Each pairof end effectors includes one unit that has a concentric drill/rivetmachine, and an opposed unit that has just an electromagnetic riveter.These units will be describe in detail below. Four pairs of opposed endeffectors 40 are shown but the number is limited only by the spaceprovided on the carrier 36. Theoretically as many pairs of opposed endeffectors 40 could be provided on the carrier as there are lines ofrivets to be placed.

The frame 32 is mounted on a pair of pivots 42 for rotation by a panelroll device 43 about a pivotal axis 44 to rotate the workpiece 34 up anddown between the opposed pairs of end effectors 40 to correctly presentit to be acted upon by them. The two pivots 42 are each mounted in anupright stanchion 46, and the radius from the pivotal axis 44 to theworkpiece is equal to the radius of curvature of the workpiece, so thespacing between the end effectors 40 and the surface of the workpiecedoes not change significantly when the workpiece is moved. Aconventional sensor (not shown) reads the angular position of the pivot42, from which the position of the workpiece 34 can be computed by acontroller 48.

As shown in FIG. 2, each of the pairs of end effectors 40 supported bythe carrier 36 includes a mounting cage 104 for movement toward and awayfrom the workpiece. The cage 104 will be described in detail below. Eachend effector 40 can be rotated on a roll device 54 about a roll axis 56to normalize the axis 109 of the end effector to the surface of theworkpiece on a plane through the pivotal axis. If desired, a wristdevice 58 can be used to rotate the end effector 40 about its own axis109. Two parallel rods 60 and 62 of an elevator 63 are mounted in thecarrier for each end effector 40, and means such as a ball screwconnection 61 between the rods and the end effector 40 provides verticaltranslation for the end effector parallel or tangent to the plane of theworkpiece when the rods are rotated by an elevator driver 64. The rods60 and 62 are also used in conjunction with the roll device 54 to alignthe opposed pairs of end effectors when the roll angle about the axis 56is changed. In this way, the workpiece and/or the end effectors 40 canbe moved to position a drill/rivet machine at a perpendicularorientation to the workpiece anywhere that a rivet is to be placed.

Movement of the carrier 36 along the machine base 37 is accomplished bythe mechanism shown in FIG. 3. The carrier 36 has a carrier plinth 66which is grooved on its underside to receive a pair of machine ways 68on which the carrier 36 is guided and supported for movement along the Xaxis, parallel to the axis of rotation 44. The carrier plinth 66 runs onroller bearings or air bearings for smooth, low friction motion undercontrol of a motive system 70. There are two separate elements of themotive system 70: a linear encoder 72, and an integrated unit 74 havinga motor, gearbox, pinion and brake. The pinion of the integrated unit 74engages a gear rack 76 located on the machine base with its gear teethfacing downward to prevent dirt from collecting in and clogging the gearteeth. The motor drives the pinion through the gearbox to move thecarrier to the right or left on the machine base 37, and when thecontroller 48 deenergizes the motor, the brake automatically stops themotor at precisely that position without coasting to a stop at someindeterminate position.

The linear encoder 72 includes a reader unit 75 and an encoded strip 76which is encoded with optical or magnetic position information that canbe read by the reader unit to provide the controller 48 with feedbackinformation concerning the position of the carrier 36 on the machinebase 37.

The system controller 48, shown schematically in FIG. 5, includes a cellcontrol 78 which controls the movement of the positioner frame 32 andthe carrier 36, and also provides signals for coordinating the functionsof an end effector controller 80 which can be physically located in thesame cabinet with the system controller 48 and which controls theoperation of the cage 104 and the drill/rivet machine of the endeffector 40. The system controller 48 will be described here, and theend effector controller 80 will be described later in connection withFIGS. 34 and 35.

A data set for fastener locations for each panel to be processed in theapparatus of FIG. 1 is converted to a part program off line and isstored in a part program data base 81 that can be loaded into the systemcontroller 48 when that part is mounted on the frame 32. The systemcontroller "knows" where the panel is on the frame by virtue of the keyindexing holes 33 in the locating ears 35 which are used to preciselyposition the panel on the frame 32. With the panel positioned correctlyon the frame 32 and the data set for the fastener locations loaded intothe controller 48, the controller can position the panel with respect tothe end effectors, and the end effectors can position themselves toaccurately drill holes, place rivets and upset the rivets within aposition tolerance of ±0.2 mm. The commands to move and orient themachine and the end effectors 40 are generated by user software 82 inthe system controller 78 and transmitted through a motion interfaceboard 83 which coordinates the commands to the machine motive devicesand confirms that the motion has been properly executed through feedbacksignals from each of the motive devices, to be explained below.

An example of a typical panel on which this machine operates is anairplane fuselage panel on which longitudinal stringers andcircumferential shear ties are riveted. For installing the shear ties,the panel is positioned with respect to the end effectors 40 primarilyusing the X-axis and panel roll axis. The end effectors would bepositioned by their yaw, roll and pitch devices to be perpendicular tothe cylindrical panel, and would normally not need to be moved since thestringers run in straight longitudinal lines parallel to the X-axis, andthe shear ties run in arcs around the panel parallel to an arcsubscribed on the panel by a radius rotated around the axis 44.

The controller 48 first moves the carrier 36 to a station file androtates the frame 32 to position the panel about the axis of rotation 44so that the end effectors 40 are correctly positioned at a shear tielocation. The end effectors 40 then clamp and drill, insert rivets, andupset the rivets as will be described in detail hereinafter. Thecircumferential spacing between the rivets in the shear ties and betweenthe stringers can be adjusted using the rods 60 and 62 and the rotationof the frame about the axis of rotation 44. In most cases, thisadjustment is be done only once for each run down the panel by thecarrier 36.

For riveting the stringers to the panel, the end effectors 40 are set tothe proper circumferential spacing between the stringers around thepanel using the rods 60 and 62 to position the end effectors on theY-axis. The panel is rolled about the axis 44 to position the endeffectors 40 on the stringer centerlines, and the end effectors areoperated as before to clamp, drill, insert rivets and upset the rivets.After each operation, the carrier is indexed down the X-axis to the nextrivet location and the process repeated until all the rivets have beenplaced. The entire panel can thus be riveted without operatorintervention whatsoever.

In the case of complex contour parts, it is sometimes necessary toreposition the pairs of end effectors individually for each rivet sinceit is necessary that the pairs of end effectors 40 each be normal to thesurface of the workpiece when drilling and riveting. This is theexceptional case, however; most riveting can be done by moving andoperating the end effectors 40 together in ganged style.

A complex contour part may occasionally require that the axis of the endeffector pairs be positioned at a slightly non perpendicular angle tothe axis of rotation 44. If the machine shown in FIG. 1 is to be used torivet such a part, another end effector motive system, shown in FIG. 4would be mounted in the carrier 36 for each pair of end effectors 40.The motive system in FIG. 4 adds an additional axis X' which enables thetwo end effectors in each pair to be moved relative to the carrier 36 inthe X-direction. In the case of a complex contour part in which thecontour varies in the X-direction, the yaw motor can be rotated inopposite directions on the two end effectors in each pair, and the rollmotors can be rotated by the amount needed to normalize the endeffectors with the panel surface. Then the X' axis motor can be operatedin opposite directions on each side to align the axes of the endeffectors so that they abut at the panel for supported drilling andriveting.

Turning now to FIG. 6, a pair of opposed end effectors 40 is shown,separated from the carrier 36. The pair of opposed end effectors 40includes a drill/rivet unit 100 and a rivet bucking unit 102. The rivetbucking unit 102 is similar to the drill/rivet unit 100, with theexception that it does not include the elements pertaining to thedrilling function, and it does not have a shuttle, all of which will bedescribed in detail below. Since the rivet bucking unit 102 is otherwisesimilar, it will be described only insofar as it differs from thedrill/river unit 100.

The drill/rivet unit 100 includes a cage 104, shown in an explodedperspective view in FIG. 7 and in partial cross section in FIG. 11. Thedrill/rivet components and the other motive and sensing components foroperating the drill/rivet components are mounted on the cage. Inaddition, the cage itself is designed to extend and retract toward andaway from the workpiece to clamp the workpiece firmly between pressurefeet to hold the parts of the workpiece clamped tightly together whilethey are being drilled and riveted by the drill/rivet components.

The cage 104 includes a top plate 106, as shown in FIGS. 7 and 9, havingthree lobes 108 spaced approximately equally around a central axis 109.Each of the lobes 108 has a hole 110 in which a plinth 112 is mounted.Each plinth 112 receives and rigidly supports a guide rod 114. A bottomplate 120 is mounted on the other end of the rods 114 by use of threecage cylinders 122 which are secured to the bottom plate 120, one eachon each of three lobes 124 arranged at approximately equal angularpositions about the central axis 109 and corresponding in position tothe lobes 108 on the top plate 106. The bottom plate 120 on both units100 and 102 can move axially along the rods 114 a limited distance toenable the machine to clamp a workpiece between pressure feet on therivet bucking unit 102 and the drill/rivet unit 100, as will bedescribed in more detail below.

A middle plate 130 is slidably mounted on the rods 114 for axialmovement between the top plate 106 and the bottom plate 120. The middleplate 130 supports the components of a drill, shown partially in FIG. 8and in FIG. 11, including a drill motor 160, a feed motor 150, and aball screw drill feed system 140, all of which are provided by EOASystems, Inc. of Dallas, Texas. Three lobes 132 on the middle plate 130correspond in position to the lobes 108 and 124 on the top plate 106 andbottom plate 120, respectively. Each of the lobes 132 has an opening 134in which is mounted a bearing 136 by which the middle plate 130 issupported on the rods 114. The position of the middle plate 130 betweenthe top plate 106 and the bottom plate 120 is controlled by the ballscrew 140, shown in FIG. 10, supported in a bearing 142 mounted on alobe 144 on the middle plate 130, and on a cylindrical receptacle 146mounted on a corner 148 on the bottom plate 120. The ball screw isdriven by the motor 150 mounted on an integral plate 152 projecting fromthe middle plate 130. The motor 150 drives the ball screw 140 through apair of cog wheels 154 and 156 on the motor 150 and lead screw 140respectively, through the agency of a cog belt 158.

The drill motor 160 is mounted on an attachment plate 162 which isfastened to the middle plate 130 by screws 164. The motor 160 drives acog wheel 166 which is shown in FIG. 11 acting through a cog belt 168 todrive another cog wheel 170 attached to a spindle 172 mounted forrotation inside a drill quill 174 mounted on the attachment plate 162.This makes it possible to dismount the entire drill unit for service orrepair by merely removing the screws 164 and lifting the plate 162 andthe attached drill motor and drill quill 174 out of the cage 104. Adrill bit 176 is mounted in a collect 178 secured to the lower end ofthe drill spindle 172 for drilling holes in the workpiece, as will bedescribed in more detail below.

Each cage cylinder 122, as shown in more detail in FIGS. 11 and 12,includes an upper extension cylinder 180 having a bearing system 179 tosupport the guide rod 114 while allowing it to move axially in the cagecylinder 122. The lower end of the cage cylinder 122 has a flange 181 bywhich the cage cylinder 122 is fastened to the bottom plate 120 byscrews 182. The lower end of the cage cylinder 122 is enlarged slightlyand bored out at the bottom to provide a cylinder 183 which receives aPiston 184 secured to the lower end of the guide rod 114. An air fitting186 is connected to the lower end of the cage cylinder 122 communicatingwith the cylinder 183 above the piston 184 for moving the cylinder andthe attached bottom plate 120 upward relative to the guide rod 114, awayfrom the workpiece. A similar fitting 188 communicates with the cylinder183 below the piston 184 for extending the cylinder 122 and the attachedbottom plate 120 downward relative to the guide rod 114, toward theworkpiece.

Each of the three cage cylinders 122 is identical to the cage cylindershown in cross section in FIGS. 11 and 12, and the fittings 186 and 188of each cylinder are each connected, respectively, to a single airsupply line controlled by a single solenoid air valve (to be describedin more detail below), so that when the lower part of the cylinder 183in each of the cage cylinders 122 is pressurized with air through theair fitting 188, all three cylinders 122 move vertically downwardsimultaneously, pushing the bottom plate 120 toward the workpiecepositioned between the opposing contact surfaces of the drill rivet unit100 and the rivet bucking unit 102, as will be described in more detailbelow.

The lower end of the drill quill 174 is laterally supported in acylindrical bearing 190 as the quill moves vertically while drilling ahole in the workpiece. The bearing 190, as best shown in FIGS. 13 and14, is mounted in an axially extending cylindrical bore 192 in a quillbearing support member 194 which is attached to the bottom plate 120with screws 198 through a lower flange 196. The quill bearing supportmember 194 has an upper flange 199 to which is bonded a rubber bumperdisc 201, and has a series of axially extending arcuate slots 200disposed concentrically around the bore 192 for receiving a segmentedupper portion 202 of a driver 203 for an electromagnetic riveter to bedescribed below. The cylindrical bearing 190 has a flange 204 whichseats on the top surface 205 of the support member 194 between the bore192 and the arcuate slots 200. The flange 204 has an outside peripheralcylindrical surface 206 which extends radically just slightly beyond theinside peripheral edge of the arcuate slots 200 and acts as a radialbearing for the segmented upper portion 202 of the driver.

As best shown in FIG. 13, an electromagnetic riveter 208 isconcentrically disposed around the drill quill 174. The upper section ofthe electromagnetic riveter 208 has a high amperage annular flat coil210 made in accordance with U.S. Pat. No. 4,146,858 or can be acommercially available coil, such as made by Electroimpact, Inc. ofSeattle, WA. The coil is potted into a recess 212 in a recoil mass 214with an epoxy potting compound 213, and is connected to a power cable205 (shown in FIG. 6.) The recoil mass 214 is secured to a support plate216 and is designed to slide axially relative to the drill quill 174 onannular phenolic sleeve bearings 215 and 217 set into upper and lowerrecesses in the recoil mass 214. A key 211 is attached to the recoilmass 214 by a screw 209 and fits into a longitudinal keyway 207 in thedrill quill 174 to guide the axial movement of the recoiling portions ofthe electromagnetic riveter and prevent the support plate 216 and itsattached electromagnetic riveter from rotating as it moves axially.

The lower half of the electromagnetic actuator has a transducer 218,which is a thick copper plate, in contact with, but electricallyinsulated from, the coil 210. When a power supply circuit, to bedescribed below, is `fired` to send a burst of high amperage currentthrough the coil 210, a rapidly changing magnetic field is generatedwhich induces strong eddy currents in the conductive material of thetransducer 218. These eddy currents in turn generate an opposingmagnetic field which creates a repulsive force between the coil 210 andthe transducer 218, causing them to separate with great force.

The upper portion 202 of the driver 203 has a small lip 219 which isused to accurately locate the transducer 218 concentrically on the topof the driver 203. The driver 203 is fastened to the transducer 218 byscrews 220 through a flange 221 on the top of the driver upper portion202. The driver 203 has a lower cylindrical portion 222, shown in FIG.14, which is fastened by screws 224 through a lower flange 223 on thetop of the driver lower portion 222 to the upper segmented portion 202.

A recess 226 is provided on the top inside edge of the lower portion 222of the driver 203 to receive an annular square ring 228 which is bondedinto the recess to provide a resilient bumper and fixes the upper limitof the travel of the driver 203. The square ring 228 is made of 90durometer nitrile to absorb and uniformly distribute the impact when thedriver is returned to its uppermost position by a pop back cylinder 230after the electromagnetic riveter has "fired".

The pop-back cylinder 230 has a piston rod 231, attached to an internalpiston, which exerts an upward force on the transducer 218 and theattached driver 203 when the pop-back cylinder 230 is pressurizedthrough an air line 232 under control of an air solenoid valve, to bedescribed below. The pop-back cylinder 230 is fastened to the bottomplate 120 by a nut 233 having a hex extension 234. The nut 233 isthreaded onto a cylindrical threaded extension 235 on the bottom of thepop-back cylinder 230 which extends into a stepped hole in the bottomplate 120 to hold the pop-back cylinder in place.

The lower end of the driver lower portion 222 is tapered inwardly at 236and terminates in a lower cylindrical portion 238 which fits snugly inand is guided by an opening 240 in a top wall 242 of a shuttle carrier244. The shuttle carrier will be described in detail below in connectionwith FIG. 15.

A recoil cylinder 246 is fastened to the bottom plate 120 by screws 248.The recoil cylinder 246 is a single acting, spring return cylindercontrolled by a pressure regulating solenoid (to be described below)delivering air to the cylinder 246 through an air line 250. A piston rod252 extends through a hole 254 in the support plate 216. A piston rodextension 253 is fastened to the piston rod 252 by a screw 258. Anenlarged head 256 on the piston rod extension 253 engages the topsurface of the support plate 216 through a 90 durometer urethene washer260 to cause the piston to compress, so that when the recoil mass 214and the attached parts of the electromagnetic riveter 208 recoilupwardly when the coil 210 is energized and forcably repelled from thetransducer 218, the plate 216 is driven upwardly, raising the piston rod252 and its attached piston in the cylinder 246, compressing gas in thecylinder 246, thereby absorbing and storing in the recoil cylinder theenergy of the recoiling portions of the electro- magnetic riveter 208when it is actuated.

A damping cylinder 262 is held in a metal sleeve 264 and clamped thereinby a clamp 265 at the lower end of the sleeve 264. The sleeve 264 anddamping cylinder 262 are clamped in a hole 266 in the support plate 216by a screw 268, threaded into the support plate 216 and squeezing a slot270 in the support plate 216 closed onto the sleeve 264. The dampingcylinder 262 has a plunger 272 which bears against the bottom plate 120and serves to damp the return motion of the recoiled portions of theelectromagnetic riveter 208 which are being returned to the startposition by the recoil cylinder 246.

The current through the coil 210 is on the order of about 5-10 kA for apulse of about 1-2 milliseconds and generates about 35 watt-hours ofresistive heating each time the coil is fired. The conventional coolingtechnique can be supplemented or replaced with a simplified coolingscheme which relies on contact and conductive heat transfer between thecoil 210 and the transducer 218. Since the transducer 218 is made ofcopper and has a high thermal conductivity, it will tend to conduct heataway from the coil when it is in contact with the lower face of thecoil, where most of the current flows and where most of the heat isgenerated. By cooling the transducer 218 instead of directly cooling thecoil 210, ordinary plant air can be used and the special drying andfiltering systems required for direct air cooling of the coil 210 arenot required.

As shown in FIG. 12, the coil cooling system includes a shroud 273around the transducer 218, and a fitting 274 for admission of coolingair to one side of the coil through a supply line 276. The other side ofthe shroud 278 has an opening 275 to permit the exhaust of cooling airwhich had been heated by heat transfer from the transducer 218 to thecooling air flow around both sides of the transducer. The transducer isprovided with radial fins 277 which extend around the entirecircumference to enhance the heat transfer from the transducer to theair flow in the space between the transducer and the shroud 273,although the fins are milled off opposite the fitting 274 and theopening 275 to facilitate air movement at the inlet and outlet of theshroud. An additional shroud 278 is screwed to the top of the shroud 273with screws 279 to protect the coil 210 from dirt or accidental contact.

A shuttle shown in exploded perspective view in FIG. 15, includes ashuttle carrier 244 which is screwed to the underside of the bottomplate by screws 280 extending through holes 281 in the top 242 of theshuttle carrier 244. The top 242 of the shuttle carrier 244 has acylindrical boss 282 which fits into a hole 284 in the bottom plate 120on the axial centerline 109 of the machine. The hole 284 is at its topend (as shown in FIG. 13) and the boss 282 is tapered with the sameslope to present a uniformly tapered hole in the base plate 120 openinginto the shuttle carrier 244. The taper angle of the hole 284 and theboss 282 is the same angle as the taper of the tapered portion 236 ofthe driver 203 so the driver 203 can have the maximum downwardtranslation without interfering with the sides of the hole 284. Thecylindrical lower portion of the driver 238 fits snugly in the opening240 in the shuttle carrier to provide a lateral bearing for the lowerend of the driver.

The cross section of the shuttle carrier 244 is generally an invertedU-shape, having two opposed side walls 286 and 288 depending from thelateral edges of the top wall 242. Each wall 286 and 288 has a shortinwardly extending flange 290 and 292 which support a shuttle body 294for longitudinal sliding movement within the shuttle carrier 244. Acircular pressure foot 296 is secured to the underside of the shuttlecarrier 244 by a pair of screws 298 (only one of which is shown in FIG.15) and the pressure foot 296 is precisely located on the axial centerline 109 of the machine by a raised rib 300 on the bottom face of theflanges 290 and 292, within which the pressure foot fits.

The shuttle body 294 is moved by an air cylinder 302 having anattachment block 301 fastened to the side 288 of the shuttle carrier 244by screws 303. The cylinder 302 has a piston rod 304 connected to an arm306 by a pivot connection 308. The arm 306 is welded to an end plate 310which is fastened to the rear end 312 of the shuttle body 294 by screws314. A stop plate 316 having a projecting ear 318 is fastened betweenthe rear end 312 of the shuttle body 294 and the end plate 310.

The ear 318 of the stop plate 316 extends through an opening 320 in thedepending side 286 of the shuttle carrier 244. The opening 320 has aflat face 322 at its rear end and a parallel flat face 324 spaced at theother end of the opening 320. A recess in the form of a circular segment326 and 328 is provided adjacent the top and bottom edges of the opening320 to receive a pin guide sleeve 340 which receives and guides a pin335 threaded to a piston 334 of a mid-stop air cylinder 330. The pinguide sleeve 340 is secured in place in the recess 326/328 by screws(not shown.) A cylindrical flange 342 on the guide sleeve 340 fits intothe circular segment recess 326/328 for accurate placement. The outerend of the pin guide sleeve 340 has a square flange 344 to which the aircylinder 330 is fastened. The cylinder 330 is a double acting cylinderhaving two air fittings 336 and 338 whereby the piston 334 can beextended or withdrawn under control of a detented double air solenoidsupplying pressurized air to one side or the other of the piston 334through the air fittings 336 and 338.

The operation of the shuttle body movement under control of the cylinder302 and the mid-stop air cylinder 330 is as follows: with the mid-stoppiston 334 retracted and the pin 335 withdrawn into the pin guide sleeve340, the cylinder 302 is pressurized through an air line 346 connectedat the remote end of the cylinder 302 to fully extend the piston rod 304and move the shuttle body to its rear or rightmost position, asillustrated in FIGS. 15 and 16, with the ear 318 against the end face322 of the opening 320. The mid-stop air cylinder 330 is pressurizedthrough the fitting 336 to extend the piston 334 so the pin 335protrudes into the path of travel of the ear 318.

When it is desired to move the shuttle body to its mid position, thecylinder 302 is depressurized through the fitting 346 and is pressurizedon the other side of the piston through the fitting 348 to cause thepiston rod 304 to move to the left in FIG. 15 to the positionillustrated in FIG. 16. This moves the shuttle body 294 to the midposition, illustrated in FIG. 17, where the ear 318 engages the pin 335.When it is desired to move the shuttle body 294 to its extreme left-mostposition, the piston 334 is retracted by pressurizing the cylinder 330through the fitting 338 and venting the Pressure through fitting 336which withdraws the piston 334 and retracts the pin 335 into the pinguide sleeve 340 where it clears the ear 318 and allows the shuttle body294 to be pushed by the piston rod 304, under the influence of the airpressure in the cylinder 302, and the arm 306 to its left-most position,as illustrated in FIG. 18, where the ear 318 contacts the forward face324 of the opening 320 to accurately locate the shuttle body at thedesired position.

The shuttle body 294 includes a pair of longitudinal recesses 350 and352 along both its bottom longitudinal edges. The recesses 350 and 352receive the flanges 290 and 292 of the shuttle carrier 294 to guide theshuttle body 294 for smooth longitudinal sliding motion within theshuttle carrier 244. The bottom face 354 of the shuttle body 294 isflush with the bottom surface of the flanges 290 and 292, so thepressure foot 296, fastened to the bottom of the shuttle carrier 244, isalso flush with the bottom surface 354 of the shuttle body 294.

A drill cavity 356 in the shuttle body 294, shown in FIGS. 15 and 24-28,includes a vertical cylindrical bore 357 opening in the top surface ofthe shuttle body 294 adjacent the front edge 358 of the shuttle body294. The very bottom of the cylindrical bore 357 is tapered inwardly at360. As shown in FIG. 24, the angle of the taper 360 corresponds to theangle of a tapered hole 362 through the pressure foot 296 to present auniform tapering bottom of the drill cavity 356 for the purpose offacilitating removal of all the drill chips created when the drill bit176 drills a hole in the workpiece.

The shuttle body 294 has a front face 364 extending downwardly andforwardly from the front edge 358. An angled opening 366 extends fromthe front face 364 through the front of the shuttle body 294 and opensinto the drill cavity 356. A fitting 368 for attachment of a vacuum hose370 is attached to the front face 364 of the shuttle body 294 by screws372. The vacuum hose 370 is connected to a source of vacuum 371 fordrawing air, mist lubrication, drill chips and long or short rivets outof the drill cavity 356 in a manner to be described below.

As shown best in FIG. 28, a downwardly angled opening 374 communicatesfrom the side of the shuttle body 294 into the drill cavity 356. Theangled opening 374 registers with an angled opening 376 in the side ofthe shuttle carrier 244 to permit an air nozzle 378 secured to theunderside of the bottom plate 120 by a clamp 380 to blow air deliveredfrom an air line 382 and blow mist lubrication metered through alubrication line 384 from a metering lubrication pump 386 into the drillcavity 356. The air from the air line 382 is controlled by an airsolenoid valve under central control of the end effector control system80, as will be described in more detail below.

The operation of the drill operating in the drill cavity 356 will now bedescribed. Under control of the end effector controller 80, to bedescribed below, the drill motor 160 is turned on and, operating throughthe cog wheel 166, the cog belt 168 and the cog wheel 170, rotates thespindle 172 supported in the drill quill 174. Likewise, the controller80 turns on the drill feed motor 150 to rotate the ball screw 140through the cog wheels 154 and 156 and the cog belt 158 to plunge themiddle plate 130 toward the bottom plate 120 to cause the drill bit 176to drill a hole in the workpiece. At the same time that the spindle isturned on, the vacuum pump 371 connected to the vacuum hose 370 isturned on and, as the drill descends through the drill cavity 356, theair through the air nozzle 378 is turned on and the lubrication pump 386meters a small amount of lubrication into the air line 382 to cause alubricating mist to be sprayed onto the drill 176 as it passes throughthe drill cavity 356. Only a small amount of lubricant is metered intothe air line 382 so that excess lubricant does not contaminate theworkpiece. Moreover, the capacity of the vacuum pump drawing air throughthe vacuum port 366 is greater than the capacity of the air line 382 sothat the drill cavity 356 is at a slightly lower air pressure and airwill be drawn in through the top opening and through the angled opening374 which then is all drawn out through the vacuum port 366 so that mistlubrication is blown only onto the drill bit 176 and then out throughthe vacuum port 366, and does not settle onto the workpiece. In thisway, distribution of the lubricant is controlled to prevent it fromcontacting the other parts of the machine or becoming entrained in theair around the work area where it could settle onto the equipment, worksurfaces or the workpiece.

As the drill drills into the workpiece, the vacuum continues to draw airthrough the vacuum port 366 and air continues to blow through the nozzle378, although no more lubricant is metered into the air line 382 afterthe initial pulse which was timed to lubricate the drill bit 176. Thedrill bit cuts into the workpiece and creates drill chips which spiralup into the drill cavity 356. As the chips break off, they are entrainedin the air swirling through the drill cavity 356 and are drawn throughthe vacuum port 366 to be deposited in a container from which they maybe periodically emptied. Since the tapered bottom 360 of the drillcavity 356 matches the size and slope of the tapered hole 362 in thepressure foot 296 at the interface therebetween when they are aligned,and since the opening in the bottom of the pressure foot 296 is nearlythe same size as the drill bit 176, there is no surface on which thedrill chips can collect, so they are all drawn up and out through thevacuum port 366. Any tendency for the drill chips to collect in thetapered opening 360 and 362 is further negated by the air blown throughthe nozzle 378 which tends to swirl the chips in the drill cavity 356 tokeep them entrained in the air in the cavity and facilitate theirremoval through the vacuum port 366.

After the hole has been drilled and countersunk, the drill feed motorreverse and withdraws the drill bit from the hole just drilled in theworkpiece and out of the drill cavity 356. The shuttle cylinder ispressurized through the air line 348 to move the shuttle body 294 to itsmidstop position, shown in FIGS. 17 and 25, with the ear 318 against thepin 335. At the midstop position of the shuttle body 294, a slot 390 inthe shuttle body 294 is aligned with the machine axis and the hole 362in the pressure foot 296. The slot 390, as shown in FIG. 15, is open atthe top of the shuttle body 294 and on the left side facing the side ofthe shuttle carrier 244 in which the midstop cylinder 334 is mounted.The slot 390 extends about two-thirds into the depth of the shuttle body294. A bore 392, shown in FIGS. 22 and 23, extends vertically through acenter section 393 in the shuttle body 294 below the slot 390, and opensin the bottom thereof. The bore 392 is counterbored at 394 to provide asmall step in which a flange 396 in the top of a rivet guide tube 398can be seated.

The slot 390 in the shuttle body 294 receives a rivet guide shoe 400which is secured in place in the slot 390 by a screw 399. The guide shoe400 receives a rivet sent from a conventional rivet blow feeder 405(shown schematically in FIGS. 17 and 30) through a rivet supply line 401to a rivet brake and staging tube 402 attached to the side 286 of theshuttle carrier 244. The rivet brake tube 402 is aligned with a hole 408in the side 286 of the shuttle carrier 244, and is fastened thereto withscrews 410. The rivet is guided by the rivet guide shoe 400 through a90° bend and into the rivet guide tube 398 through which it is insertedinto the hole drilled into the workpiece.

The rivet guide shoe 400 through the slot 412, as shown in FIGS. 15, 19,20, 22, and 23, is a rectangular block 403 of aluminum having aprojecting arm 404. A slot 406 is machined in the block 403 in the formof a quarter sector of a circle opening into the bottom and side of theblock 403, and the inside surface of the slot 406 is coated with PTFEimpregnated aluminum oxide to give a tough, low friction surface onwhich the rivets can glide smoothly around the slot when they are beingfed into the holes in the workpiece. The slot 406 opens in the block 403on the side facing the side 286 of the shuttle carrier 244 and, when theshuttle body 294 is in its rivet insert position, aligns with the hole408 in the side 286 of the shuttle carrier 244. The slot 406 has arounded or arched outside periphery to ensure stable travel of the rivetaround the 90° bend of the slot 406. The outside periphery of the slot406 at the bottom side of the block 403 has a cylindrical recess 411 toreceive the top end of the flange 396 on the rivet guide tube 398 toensure exact alignment of the rivet guide tube with the slot 406.

A slot 412 is cut horizontally into the rear end 312 of the shuttle body294 just above the bottom edge 414 of the rear end 312 and extends allthe way through the shuttle body 294 into the center section 393 belowthe recess 390. The slot 412 intersects both a rivet die cavity 415,shown in FIGS. 22, 24-27, and 29, and the bore 392. The slot 412 ends inan end portion 416 which forms an air plenum around the rivet guide tube398 in the bore 392. The bore 392 is counterbored to produce a largerdiameter lower portion 418 of the bore 392 approximately equal indiameter to the diameter of the tapered hole 362 in the pressure foot296 at its top periphery. An annular space 420 thus exists between theenlarged diameter lower portion 418 of the bore 392 and the outsidesurface of the rivet guide tube 398 through which an annular curtain ofair enters from the plenum 416 into the tapered hole 362 in the pressurefoot 296 to guide a rivet through the hole 362, as will be describedbelow.

The operation of the rivet feeding station of the shuttle is as follows:A rivet is sent by the rivet blow feeder 405 through the rivet supplyline 401. The rivet is fed from the blow feeder while the shuttle body294 is in its leftmost or drilling position, shown in FIGS. 16, 24, and28, with the drill cavity 356 aligned with the machine axis. In thisposition, the arm 404 on the guide shoe 400, lying in a recess 422 inthe side of the shuttle body 294, lies in line with and blocks the endof the rivet brake and staging tube 402 thereby forming an effectivedead end in that tube. It is advantageous for the rivet blow feeder tofeed the rivet through the rivet supply line 401 at high speed so thatit can be staged and waiting at the arm 404 when the rivet feedingstation of the shuttle is ready to receive the rivet, so no time is lostwaiting for the rivet to arrive. However, it is necessary to slow therivet velocity before it reaches the shuttle to prevent impact damage tothe rivet when it hits the arm 404. To accomplish the high speed feedrate and the low speed arrival rate, the rivet brake and staging tube402 has a series of holes 424 drilled laterally through the tube so aircan exit the tube and an air flow be established to convey the rivetfrom the rivet blow feeder to the rivet brake and staging tube. When therivet arrives at the tube 402, it runs into an air cushion created bythe arm 404 blocking the end of the tube 402. The air cushion slows therivet velocity so that it comes to a gentle rest against the arm 404.The holes 424 in the rivet brake and staging tube permit a flow of airbehind the staged rivet at the arm 404 which provides the dynamicpressure to hold the rivet in place against the arm 404 until it isneeded in the rivet feed station.

The rivet guide shoe 400 is coated with a tough, low friction coatingsuch as teflon impregnated aluminum oxide so that when the shuttle movesand carries the arm 404 of the rivet guide shoe 400 across the end ofthe rivet brake and staging tube 402, it produces very low friction andno significant wear to the guide shoe or to the rivet.

After the hole has been drilled in the workpiece 34 and the drillretracted, the shuttle body 294 is moved leftward to the center positionby pressurizing the cylinder 302 through the air line 348 and whichcauses the shuttle body to shift to the left in FIG. 15 until the ear318 on the plate 316 engages the pin 335 on the midstop cylinder. Atthis position, the slot 406 in the rivet guide shoe 400 is aligned withthe rivet brake and staging tube 402, as shown in FIGS. 17 and 20, andthe air pressure in the rivet supply line 401 blows the rivet throughthe end of the rivet brake and staging tube 402 into the slot 406.Because of the holes 424 in the tube 402, the air pressure acting on therivet to blow it into the slot 406 is very low, viz. on the order of 1psi, so the rivet does not gain enough speed in the rivet guide shoe 400to damage the workpiece as it enters the hole in the workpiece.

The rivet enters the slot 406 and, held against the circular, archedperiphery of the slot by centrifugal force, slides around the surfaceand enters the rivet guide tube 398. The rivet guide tube 398 has araised ear 426, shown in FIGS. 21 and 23, which has a curved top surface427 curving from a top peripheral lip 428 to taper and blend smoothlywith a bore 430 of the rivet guide tube 398. The width of the ear 426 isexactly the width of the slot 406, and the diameter of the bore 430 ofthe rivet guide tube is the same as the width of the slot 406 so thatthe rivet is guided into the rivet guide tube 398 without interferencefrom any edges or obstructions of any kind. The rivet passes into therivet guide tube 398 whose length is at least about ninety percent ofthe rivet length to stabilize the rivet and remove any angular momentumit may have when it enters the guide tube 398, so that when it passesout of the end of the tube 398 it will travel in a straight line withoutturning or rotating.

When the rivet nose exits from the end of the rivet guide tube 398, itenters the tapered opening 362 in the pressure foot 296. The sides ofthe opening 362 are too far apart to give any guidance to the nose ofthe rivet. Because the rivet head is larger in diameter than the leadingnose, it is unstable when the pressure behind the head is greater thanthe pressure in front of the nose. This would normally cause the rivetto rotate within the confines of the rivet guide tube. After exitingfrom the rivet guide tube, the rotating rivet would strike the taperedhole 362 in the pressure foot 296, or the countersink of the holedrilled in the workpiece at an arbitrary point and possibly jamdiagonally. If the rivet nose were to contact the countersink in thismanner, it would be acted upon by the pressure differential, forcing thenose increasingly harder against the countersink as the airflowtransitioned from dynamic to static. This would serve to place theentire pressure differential across the rivet with no line loss.

Accordingly, means are provided to guide the nose of the rivet straightinto the hole in the workpiece and counteract any tendency of the rivettail to drift off of the centerline 109 as it passes through the opening362. This guide means is the air curtain blowing downward through theannular gap 420. The air curtain blows straight down through the gap 420and then is converged inwardly toward the rivet shank by the convergingsides of the hole 362. If the rivet tail starts to drift off centertoward one side of the hole 362, the dynamic air pressure from the aircurtain impinging against the side of the rivet shank closest to thatside of the hole 362 will be greater than on the opposite side of therivet shank, tending to straighten it back toward the centerline 109.Moreover, if the rivet is canted to one side in the rivet guide tube398, there will be an air flow restriction on that side which results ina high pressure area on that side of the rivet relative to the oppositeside, creating a force which tends to straighten the rivet along theaxis of the rivet guide tube. Finally, there is a pressure drop from theair flow through the rivet hole resulting from suction applied, asdiscussed in more detail below, and shown in FIG. 2B, through abuck-side pressure nose 454 and the annular air flow creates a pressurein the cavity between the countersink and the rivet head. By adjustingthe annular air flow, the cavity pressure at the moment the rivet hangson the countersink can be such that the pressure differential actingacross the length of the rivet is greatly reduced. This serves toenhance the effect described above by reducing the normal force at therivet nose which results in reduced frictional resistance.

The vacuum source on the buck side unit 102 acts on the nose of therivet as its brought in line with the hole in the workpiece 34 andserves to provide the required pressure differential to draw the rivetinto the hole. It also greatly increases the annular air flow which isused before reaching the cavity pressure defined above which results ina greatly enhanced effect.

The converging annular curtain of air through the gap 420 is provided byair blown from the air plenum 416 through the slot 412, as shown in FIG.15, between the end of the rivet guide tube 398 and the enlarged lowerend 418 of the bore 392. The air enters the air plenum 416 from an airline 432 connected to a fitting 434 attached to the end plate 310. Airis delivered under pressure to the air line 432 from a connection (notshown) with the air line 348 to the retract side of the shuttle bodycylinder 302 so that air is delivered to the air line 432 whenever theretract side of the air cylinder 302 is pressurized. Air is suppliedthrough the fitting 434 on the end plate and a corresponding hole 436 inthe stop plate 316 and hence into the slot 412 milled through theshuttle body 294.

After the rivet has been installed in the hole drilled in the workpiece,the shuttle body 294 is advanced to align the die cavity 415 with themachine axis. The shuttle body 294 is advanced by pressurizing themidstop cylinder 330 through the fitting 336 to withdraw the piston 334and pull the pin 335 back into the pin guide sleeve 340 which allows thepressure in the air cylinder 302 to push the shuttle body 294 to itsforwardmost position, shown in FIGS. 18 and 27, with the ear 318 of thestop plate 316 against the front face 324 of the opening 320 in the side286 of the shuttle carrier 244.

A rivet die 440, shown in FIGS. 15 and 29, is mounted in the rivet diecavity 415. The rivet die 440 has a cylindrical skirt 442 attached tothe die near the top end thereof and depending downwardly in concentricrelationship to the axis of the die 440. The outside diameter of theskirt 442 is just slightly smaller than the diameter of the rivet diecavity 415 and functions to guide the die to slide smoothly on the axialcenterline 109 and ensure that it does not bind or become canted in thedie cavity 415.

An annular groove 443 in the die cavity 415 near the top end thereofreceives a snap ring 444 which cooperates with a cylindrical recess 446at the top outside peripheral edge of the die 440 to hold the die fromupward movement above the top surface of the shuttle body 294. The dieis urged upwardly by a helical coil spring 448 so that the normalposition of the die is at its uppermost position, with the shoulder ofthe recess 446 in contact with the snap ring 444 and the top surface ofthe die 440 flush with the top surface of the shuttle body 294. The dieis proportioned lengthwise to position a nose 450 of the die 440 justabove the lower surface of the shuttle body 294 to clear the pressurefoot 296 when the shuttle body 294 moves.

In operation, the die 440 is carried by the shuttle body 294 intoalignment with the machine axis when the pin 335 is withdrawn by thecylinder 330 to allow the shuttle body 294 to move to its fully leftmostposition in FIG. 15, to the position shown in FIGS. 18, 27, and 29. Atthat position, the die 440 in the die cavity 415 is aligned with themachine axis 109 and the driver lower cylindrical portion 238. When arivet is to be upset, the pop back cylinder 230 is depressurized toallow the recoil cylinder 246 to carry the electromagnetic riveter downas shown in FIG. 27, so that the lower end of the lower cylindricalportion 238 of the driver 203 is in contact with the top surface of therivet die 440. The downward force exerted by the recoil cylinders 246also compresses the coil spring 448 to allow the die 440 to descendthrough the pressure foot 296 into contact with the rivet. At this pointthere is now hard contact all the way from the coil 210, through thetransducer 218, the driver 203 and the rivet die 440 to the rivet.Momentarily later, to ensure that the rivet is seated first by theheaded side die, the buck side die 520 is brought into contact with thenose of the rivet, and the corresponding structure from theelectromagnetic riveter on the buckside unit is similarly in hardcontact with the nose of the rivet. The machine is now ready to upsetthe rivet.

The electromagnetic riveters on both units are now actuated to produce aforce stroke against the rivet dies on both sides of the rivet to upsetthe rivet. The timing of the force stroke on both sides of the rivet andthe magnitude and duration of the force stroke on the two sides areadjustable so as to produce the optimum force pulse on each side of therivet, optimized for the particular rivet being upset. Rivets ofdifferent kinds and different kinds of workpieces sometimes will requiredifferent combinations of timing, magnitude and duration of the forcepulse on the dies on the two sides of the rivet, and this will bediscussed in more detail below.

Turning now to FIG. 30, the pneumatic actuating system for control ofthe pneumatic motive elements in the machine is shown schematically. Allof the pneumatic elements are powered by air pressure provided by an airpressure source 456 such as plant air existing in the plant in which theriveter is to be installed. The air pressure source 456 is connected inparallel through an air supply line 458 to a bank of solenoid air valves460 which control the flow of air to the pneumatic elements in themachine. The valves 460 are solenoid operated under the control of theend effector controller 80, to be described below.

The flow of cooling air through the air line 276 on the drill rivet unit100 is controlled by a spring return, single solenoid 462, and thecorresponding cooling air flow to the coil on the rivet buck unit 102 iscontrolled by a similar spring return single solenoid 464. The airpressure to the recoil cylinder 246 is controlled by a similar springreturn single solenoid 466 on the drill/rivet unit 100, and a springreturn single solenoid 468 on the rivet buck unit 102. These coilcooling and recoil air solenoid valves are always on or open to provideair flow whenever the riveter is turned on. Likewise, the vacuum source371 is always turned on when the unit is on, except if a malfunctionoccurs and the vacuum is not needed in which case it is timed to turnoff in about four seconds.

Air supply to the air line 232 for the pop back cylinders 230 iscontrolled by a detented double solenoid 470, and the correspondingfunction on the rivet buck unit 102 is provided by a similar detenteddouble solenoid 472. The detented double solenoids are used on the popback cylinders to insure that, if power is lost, the solenoid willremain in its detented position so that when power is restored to themachine the position of the pop back cylinder will remain as it was whenthe power went off. This facilitates the initialization of the machineand prevents the various moving parts from moving into interferingpositions with respect to each other.

The cage cylinders 122 are controlled by two separate solenoids. Airflow to the retract side of the cage cylinders through the air line 186is controlled by a spring return single solenoid 474, and air flow tothe cage extend side of the cylinders 122 through the air line 188 iscontrolled by a spring return single solenoid 476. Similarly, the cageretract function on the rivet buck side 102 is controlled by a springreturn single solenoid 478 and the cage extend air flow is controlled bya spring return single solenoid air valve 480.

Air flow to the pneumatic motive elements for the shuttle is controlledby a detented double solenoid 482 for selectively pressurizing theshuttle body cylinder 302 through air lines 346 to move the shuttle bodyto the extreme right most position in which the drill cavity 356 isaligned with a machine axis 109, or toward the extreme left mostposition when pressurized through air line 348 to move the shuttle bodyto its left most position in which the rivet die cavity 415 is alignedwith the machine axis 109. Another detented double solenoid 484 controlsthe air flow to the air lines 336 and 338 of the mid stop cylinder 330.

The air through the air nozzle 378 provided through air line 382 iscontrolled by a spring return single solenoid 486. The lubricationmetering pump 386 is triggered and powered by the air pressure in theline 382 and pulses once when the solenoid 486 is activated. Themetering pump 386 does not inject another drop of lubricant until thesolenoid 486 is turned off and then turned on again.

The air to the rivet supply line 401 which conveys rivets from the rivetblow feeder 405 is controlled by an air solenoid 488 incorporated in therivet blow feeder 405 as part of the conventional device.

The sensors which provide feedback to the end effector controller 80about the position and other conditions of the end effectors 40 areshown in FIG. 31. This sensor suite includes a feed limit switch 490having a vane 492 attached to the middle plate 130 and a series of threehall effect switches 494 to sense the presence of the vane and therebyindicate when the middle plate 130 has arrived at the three positions,for a purpose that will be explained below. A thrust sensor 496 ispositioned at the top of the ball screw receptacle 146 to sense thethrust exerted by the ball screw 140, and hence the pressure exerted bythe point of the drill bit 176 on the workpiece. A linear potentiometer498 is attached to the top plate 106 and has a plunger attached to thecage cylinder 122 for measuring the extension of the cage. A shuttleposition switch 500 includes a vane 502 and three hall effect switches504 positioned to register the presence of the vane when the shuttlebody, to which the vane is attached, moves to the drill, rivet insert,and rivet position respectively.

A proximity switch 506 is positioned around the rivet supply tube 401for detecting and signaling when a rivet passes through the tube on itsway to its staged position in the rivet brake and staging tube 402.

The rivet buck unit 102 has a cage linear potentiometer 507 which isidentical to the linear potentiometer 498 for the drill/rivet unit 100and performs the same function. The rivet buck unit does not have a feedlimit switch like the switch 490 on the drill/rivet unit 100 since itdoes not have the middle plate 130 and the drilling equipment supportedby the middle plate 130 in the drill/rivet unit 100. Similarly, therivet buck unit 102 does not have a shuttle and so lacks the shuttleposition sensor 500 which is on the drill/rivet unit 100. A gravitysensor 508 is fastened to one of the recoil cylinders 246 on thedrill/rivet unit 100, and a similar gravity sensor 509 is fastened toone of the recoil cylinders on the rivet buck unit 102. The gravitysensors 508 and 509 provide information to the end effector controller80 regarding the orientation of the machine regulator to the directionof gravity. The end effect controller 80 uses this information to adjustthe pressure regulator in the solenoids 466 and 468 to account for thechanges in weight hanging on the recoil cylinder when the machine ismoved by the robot 30 to change the orientation of the machine relativeto the direction of gravity. Since the pressure in the recoil cylindersis adjusted to provide the desired stroke and recoil force when theelectromagnetic riveter is actuated, and also to provide thedifferential force which insures that the rivet is held in itscountersunk hole on the drill/rivet side while the length of the rivetis measured by a rivet length sensor 510 on the rivet buck side, it isdesirable that the proper force differential be maintained despite thechange in orientation of the machine. In the absence of an adjustment ofair pressure in the recoil cylinders, it would be necessary to build ina larger pressure differential to account for the changes in forceexerted on the recoil cylinders and if that were done it would be moredifficult to optimize the recoil cylinder pressure to achieve thedesired recoil stroke and recoil force during the recoil of theelectromagnetic riveter.

The rivet length sensor 510, shown schematically in FIG. 31, has twohall effect switches 512 and 514 mounted on a flat area 516 milled ontothe outside wall of the quill bearing support member 194. A vane 518 isfastened to the transducer shroud 273 and the length of the vane 518 andthe position of the hall effect switches 512 and 514 is arranged toindicate if a rivet inserted into a hole in the workpiece is too long ortoo short. This is accomplished in the normal riveting cycle andoperates as follows. After the rivet is inserted into the hole in theworkpiece by the shuttle at the insert position, the shuttle moves tothe rivet position as shown in FIG. 29. The pop back cylinders 230 aredepressurized and the recoil cylinders 246 pull the electromagneticriveter down to press the lower cylindrical end 248 of the driveragainst the die 440. This compresses the spring 448 and presses the nose450 of the die 440 against the top of the rivet, as shown in FIG. 27.

The pop back cylinders on the buck side unit 102 are likewisedepressurized, allowing the recoil cylinders to pull the electromagneticriveter toward the bottom plate to push the lower cylindrical portion ofthe driver, in which a rivet die 520 is mounted, through the pressurenose 454 and against the nose of the rivet. The pressure in the recoilcylinder on the rivet buck side unit 102 is less than the pressure inthe recoil cylinders in the drill/rivet unit 100 so that the rivet die440 on the drill/rivet 100 holds the rivet seated in a fixed referenceposition which, in this case, is the counter- sunk hole drilled in theworkpiece 34. Thus, the axial position of the rivet die 520 in the buckside unit 102, when in contact with the nose of the rivet in theworkpiece 34, accurately reflects the axial extension of the rivetthrough the workpiece 34. By measuring the axial position of the driveron the buck side unit 102, it is possible to get an accurate measurementof the length of the rivet in the workpiece 34 held seated in thecountersunk hole by the rivet die 440 on the drill/rivet unit 100.

The two Hall effect switches 512 and 514 are spaced apart a distanceequal to the length tolerance for the rivet. Thus, if the vane 518 failsto reach the first Hall effect switch 512, that indicates that the rivetis too long because the long rivet stops the rivet die and theassociated driver and transducer on the buck side unit 102 fromextending far enough to carry the vane 518 into the Hall effect switch512. When the vane reaches the Hall effect switch 512 but has notreached the Hall effect switch 514, that indicates that the rivet iswithin the required tolerance. When the vane 518 reaches the Hall effectswitch 514, that indicates that the rivet is too short, that is, fallsoutside of the length tolerance of the rivet, because it has allowed therivet die and associated driver and transducer on the buck side 102 toextend far enough to reach the Hall effect switch 514.

A typical length tolerance for a rivet would be on the order of 0.060".With a length tolerance that small, it is convenient to mount the halleffect switches 514 and 512 at equal axial positions on the quillbearing support 194, as shown in FIG. 31, and use two vanes of differentlength, differing in length by the permissible length tolerance of therivet. In this arrangement, a rivet within length tolerance would beregistered when the long vane would have entered the hall effect switch512, but the short vane had not entered the Hall effect switch 514.

When a rivet that is outside the length tolerance is sensed, it must beremoved from the hole in the workpiece 34 and replaced with a properlysized rivet, because a long or short rivet will not produce anacceptable rivet when upset by the riveter. The rivet recovery sequenceis described below.

The operation of the machine will now be described in connection withthe system logic schematic in FIGS. 32 A-F. When the machine is firstturned on, the system software initializes the components by moving themin a predetermined sequence to a position in which the cycle may bestarted without danger of the components clashing by moving intointerfering positions. In initialization, the cage 104 is retracted, themiddle plate 130 is withdrawn to its uppermost position, the recoil andpop back cylinders are pressurized, and the pistons in both the shuttlecylinder and then the shuttle midstop cylinder are extended, in order.

With the components initialized, the machine executes a routine to teachthe end effector controller what the revolution number and angle of thedrill feed ball screw is when the tip of the drill bit is in contactwith the plane of the workpiece surface. To accomplish this, the cages104 are extended to clamp the part and the drill spindle is advanceduntil the drill bit contacts the workpiece, as indicated by the thrustsensor 496. The feed motor is then reversed and retracts the drillspindle until the vane 492 enters the center Hall effect switch 492c.The number of rotations of the feed screw is counted in a spindlerotation counter and angular position indicator (not shown) in the drillmotor, and that number is recorded in the memory of the end effectorcontroller 80, so the controller 80 knows exactly how far it is fromthat reference position to the workpiece. Likewise, the depth of thecountersink can be preset by clamping a coupon with a countersink of thecorrect depth, and repeating the above procedure. The drill is theninstructed to drill to that same depth (i.e. the same number ofrevolutions of the ball screw and to the same angle on the lastrevolution) to redrill to the same countersink depth.

The distance (i.e. the number of revolutions of the drill feed ballscrew 140) from the plane of the workpiece is already recorded in thecontroller memory, so the controller 80 can instruct the feed motor toretract the drill bit to a "ready" position in which the tip of thedrill bit lies just above the plane of the top surface of the shuttlebody 294. At this position, the drill bit does not interfere with theshuttle body when it slides from the drill position to the rivet insertposition and rivet upset position, but it is close to the workpiece tominimize cycle time.

The top and bottom Hall effect switches 494t and 494b are "soft" upperand lower limit positions for the drill, to prevent it from retractingor extending to, and jamming against, the top or bottom structurallimits of the equipment.

After the components are initialized the cycle is started. At cyclestart, the charging of the capacitors of the electromagnetic riveterpower circuit is started, as will be described in more detail below inconnection with the machine control system. Simultaneously, the spindleis turned on and the feed motor is energized to rapidly advance themiddle plate toward the bottom plate. The vacuum is turned on and theshuttle midstop cylinder is pressurized to extend the midstop pin 335. Amessage is sent to the rivet blow feeder to send a rivet through rivetsupply line 401 toward the shuttle.

A series of adjustable delays is built into the controlling software toprovide a time period in which certain actions can take place. This is acontrol methodology which assumes that, once initiated, a process oraction will be carried out within a certain maximum time period, and ifthat action is not completed within the given time period then a problemis presumed. The EMR charging circuit is given about four seconds toreach full charge, although it normally takes less than three seconds.If the capacitors have not been fully charged within four seconds, anerror message is generated by the software and the machine is shut downto permit the operator to determine what has happened.

The cage extend command initiates a timed period of about one second inwhich the cage is given to extend and contact the part. The end effectorcontroller 80 recognizes part contact when the linear potentiometer 489on both the drill rivet unit and the buck side unit indicate fullextension to the location of the part. If the full extension signal isnot received by the end effector controller 80 from the linearpotentiometers 489, an error message is generated and the machine shutsdown for service by the operator. A time period is initiated after therivet blow feeder is directed to send a rivet to allow the rivet totravel the length of the rivet supply line 401 and be registered on therivet proximity switch 506. If the rivet is not sensed within the settime period, the machine automatically shuts down for intervention bythe operator.

The cage is given a certain amount of time to settle after contactingthe part, and then mist lubrication air flow is turned on by energizingthe solenoid 486 which actuates the metering pump 386 to inject a dropof lubricant into the air line 382 to blow mist lubricant into the drillcavity 356 and onto the drill bit 186 which is parked at the lower endof the drill cavity 356. The drill feed motor is then turned on again toadvance the drill at drilling speed toward the workpiece. As the drillbit drills into the workpiece, the thrust sensor 496 is monitored by theend effector controller 80 to insure that the thrust level registerswithin a predetermined range. A thrust that is too low indicates thatthe drill bit is broken and is not contacting the part. A thrust that istoo high indicates that the bit is dull and is not cutting fast enough.In either case, a message is recorded on the system monitor for theoperator to investigate.

After the hole and countersink are drilled the system may bepreprogrammed to dwell at depth to insure that the countersink isdrilled cleanly. If the drill feed system is under damped for maximumdrilling speed, the dwell at depth feature may be dispensed with sincethe drill bit would have slightly overshot the lower most staticposition and would merely turn in the air at the dwell position. If thesystem is over damped the dwell at depth feature will result in a verysmoothly finished countersink.

While the hole is being drilled, the vacuum and mist air is on to removethe drill chips from the drill cavity. The lubrication only flowsmomentarily, but the air through the line 382 continues to blow to swirlthe chips in the drill cavity 356 to facilitate their suction throughthe suction port 366. At the end of the drilling sequence, the mist airis turned off by deenergizing the solenoid 486, but the vacuum ismaintained on so that the rivet will be drawn into the hole drilled bythe drill in the workpiece when the rivet is feed to the rivet feedingstation, as will be described below.

The drill is now retracted to its home position and, when the revolutioncounter in the drill motor recognizes that it has reached home position,the end effector controller 80 operates the air solenoid 482 to deliverpressurized air to the opposite end of the shuttle body cylinder 302through the air line 348 to retract the piston and the piston rod 304into the cylinder 302 and move the shuttle body 294 to the limitprovided by engagement of the ear 318 against the midstop pin 335. Whenthe shuttle body moves to the rivet insert position, the vane 502 on theshuttle body moves into the center Hall effect switch 504c and indicatesto the end effector controller 80 that the shuttle body is at the insertposition.

At the insert position, the rivet guide tube 398 in the rivet guide shoe400 is aligned with the machine axis 109 and the rivet guide chute 406in the rivet guide shoe 400 is aligned with the rivet brake and stagingtube 402. The rivet, which has been staged at the end of the rivet brakeand staging tube 402, is now propelled by the low air pressure at theend of the tube 402 into and around the rivet chute 406, through therivet guide tube 398, and across the gap through the pressure foot 296.As it jumps through the hole 362 in the pressure foot 296, the rivet isguided by the annular air curtain blowing through the gap 420 at the endof the guide tube 398, and is drawn into and seated in the countersunkhole in the workpiece 34 by the vacuum in the pressure nose 454 on therivet buck side unit 102.

The system controller sets a time period to allow the rivet to travelfrom its staged position at the arm 404 through the rivet guide shoe andinto the drilled hole in the workpiece. At the end of this time period,the end effector controller 80 operates the solenoid 484 to pressurizethe cylinder 330 through the air line 336 to withdraw the pin 335 andallow the cylinder 302, already pressurized from the previous step, tomove the shuttle body 294 to its left most or forward position in whichthe rivet die cavity 415 is aligned with the machine axis 109. At thatposition, the vane 502 moves into the left most switch 504L and thecontroller recognizes that the shuttle body is in the correct positionfor upsetting the rivet installed during the rivet insert step. If theshuttle fails to move to the rivet position within a time period set inthe controller software, typically about 0.3 seconds, an error messageis generated and the machine shuts down for operator correction of thefailure.

With the shuttle at the rivet upset position, the pop back cylinders 230are depressurized by the solenoid 470 which cuts off the supply ofpressurized air to the pop-back cylinders and vents the cylinders toroom air pressure. The recoil cylinders 246 can now pull the EMR support216 plate down toward the bottom plate 120, bringing the driver lowercylindrical portion 238 down against the die 440 and holding it in areference position, viz. pressing the head of the rivet in thecountersunk hole in the workpiece 34. At the same time, the pop backcylinders on the buck side unit 102 are likewise depressurized in thesame manner and the rivet die 520 in the pressure nose 454 moves againstthe nose of the rivet where it is stopped by the higher air pressure inthe recoil cylinders 246 on the drill rivet unit 100 acting through therivet die 440 on the rivet head.

The end effector controller 80 at this time recognizes the position ofthe vane 518 of the rivet length sensor relative to the Hall effectswitches 512 and 514 to determine whether the rivet length is withintolerance. If it is not, the system controller initiates a rivetrecovery sequence.

The rivet length sensor can be used with other types of rivets, as wellas with the headed rivets discussed herein. Slug rivets, for example,which do not have heads at all, can be measured by establishing areference position on one side of the workpiece, at which one end of therivet can be located, and then measuring the length of the rivet withthe other rivet die in the same manner as described above. The referenceposition can be established by, for example, locating the rivet die atsome fixed, known position and using the rivet die surface as thereference position.

The rivet recovery sequence is illustrated in FIGS. 32E and 33. In FIG.33A, the machine is shown schematically with both dies 440 and 520 incontact with the rivet. The die 440 presses the rivet into thecountersunk hole in the workpiece 34 and the rivet measurement is madeby the rivet length sensor 510 on the rivet buck unit 100. Assuming thatthe rivet was either too long or too short, the end effector controller80 recognizes an error and initiates the rivet recovery sequence. Thefirst step of the rivet recovery sequence, illustrated in FIG. 33B, isto retract the die 520 away from the rivet on the rivet buck unit 100 byextending the pop back cylinder on that side, while maintaining thevacuum to the pressure nose 454 to hold the rivet in place in thecountersunk hole in the workpiece 34.

As shown in FIG. 33C, the second step is to retract the die 440 on thedrill/rivet side by extending the pop back cylinders 230 on that side.While the vacuum in the pressure nose 454 holds the rivet in place, theshuttle is moved to the drill position in which the drill cavity isaligned with the machine axis, as shown in FIG. 33D. A rivet is sentfrom the rivet blow feeder 405 and is staged in the rivet brake andstaging tube 402.

Meanwhile, the buck side die 520 is extended, as shown in FIG. 33E, bydepressurizing the pop-back cylinder on the buck side unit to push therivet out of the countersunk hole in the workpiece 34. The rivet die520, at its fully extended position, blocks the hole in the workpiece 34so that the vacuum applied to the pressure nose 454 no longer acts topull the rivet downward into the hole. In its extended position shown inFIG. 33E, the rivet is easily withdrawn by the vacuum applied to thevacuum port 366 and the rivet is sucked out through the vacuum port anddisposed of in the chip bin. The buck side rivet die 520 is nowretracted by extending the pop back cylinder on that side. The shuttlebody 294 is moved to the rivet insert position by pressurizing thecylinder 302 through the air line 348 and retracting the piston rod 304until the ear 318 engages the stop pin 335, and the new rivet, sent fromthe rivet blow feeder and waiting at the end of the rivet brake andstaging tube 402 against the arm 404, is fed into the hole 34 whereuponthe cycle is resumed.

After the end effector controller 80 has checked the rivet length toinsure that the new rivet is within length tolerance, the systemcontroller sends a message to the electromagnetic riveter controller530, shown schematically in FIG. 35, to initiate or "fire" the actuatorin the electromagnetic riveter. The controller 530 for theelectromagnetic riveter will be described below.

After the rivet has been upset by the electromagnetic actuators drivingthe rivet dies 520 and 440 against the nose and head of the rivet, thepop back cylinders on the drill/rivet unit 102 and the rivet buck unit100 are extended to allow the spring 448 to lift the die 440 to the topof the rivet die cavity 415, and retract the buck side rivet die 520into the pressure nose 454. A delay of about 0.25 seconds is programmedinto the program software to provide sufficient time for the recoil andrebound of the electromagnetic riveter to subside before the clampingpressure of the cage is released, to prevent any damage to the workpiecethat could be caused by the movement of the pressure foot 296 while thecage is unclamping.

The cages on both the drill rivet unit 102 and the rivet buck unit 100are now retracted by pressurizing the cage cylinders 122 through thelines 186 to release the clamping pressure on the workpiece. When theend effector controller 80 recognizes that the cages are retracted, by asignal from the linear potentiometers 498 and 507, end effectorcontroller 80 signals that the riveting operation is complete, and thesystem controller 48 sends a message to the positioner 30 to shift theposition of the part and or the carrier 36 to reposition the endeffectors 40 to the new position on the workpiece where the next set ofrivets is to be installed. The cycle is then started again.

The end effector controller 80, shown schematically in FIG. 34, is a PCprogrammed to recognize start commands from the system controller 48 andthe inputs from the sensors. It is programmed to control the sequence ofoperations described in the logic diagram of FIG. 32A-F and to providesignals to the electromagnetic riveter controller 530 to coordinate theactions of the electromagnetic riveter with the operation of the drillrivet machine.

Turning now to FIG. 35, the controller 530 for the electromagneticriveter is shown having a logic unit 532 which is a programmablecontroller. The software in the controller permits the setting of thesystem parameters which include time delay, the voltage set point, andthe capacitance of the two capacitor banks for power supplies of the twoelectromagnetic actuators. The capacitance adjustment is accomplished byan SCR switching system that connects to coil 210 of the electromagneticactuator to the desired one or more capacitors in the capacitor bank, toachieve the desired capacitance within the range of capacitors in thecapacitor bank.

The voltage set point, which can be individually set for each powersupply, sets the voltage at which the capacitors are charged. Thetemperature algorithm in the logic unit computes the amount by which thevoltage must be increased to account for the increased resistance of thecoil 210 as it gets hotter. A thermocouple in the coil informs the logicunit 532 of the coil temperature, and the logic unit computes thevoltage increment that must be added to the set voltage to account forthe increase in resistance of the coil 210 that accompanies the increasein temperature, to ensure that the power which the coil can actuallyconvert to energy in the magnetic field remains the same despite thetemperature increase.

The delay is a time delay between the fire commands issued to the powersupplies for the two electromagnetic actuators. By imposing a shortdelay in the fire command to the power supply on the drill rivet side,the electromagnetic actuator on the buck side unit can begin upsettingthe rivet, using the rivet die and its electromagnetic actuator as arivet bucking bar, before the rivet die 440 on the drill rivet sidebegins its force stroke on the headed side of the rivet. This delayallows the force exerted on the nose of the rivet to begin deforming theshank of the rivet at the nose end so the shank begins expanding to fillthe hole in the workpiece at the lower or nose end of the workpiece. Therivet die 440, after the short time delay, then begins its power strokeand the force it exerts is concentrated on the head of the rivet, ratherthan being dissipated through the rivet head and into the workpiecethrough the countersunk hole. The force pulse of the rivet die 440 isthus effective to expand the shank of the rivet and then expand the headof the rivet to seal the head in the counterbore of the hole in theworkpiece 34. Without the time delay there is a tendency for the forceexerted by the rivet die 440 to be dissipated through the head of therivet and into the workpiece so that the force exerted by the die 440 isless effective in upsetting the shank at the head of the rivet and inupsetting the head itself to seal the head in the counterbore. The timedelay varies with the thickness of the rivet and the mass of the driverand the transducer, but a typical delay would be on the order of 100 to400 microseconds. The best way we have found to determine the properdelay is to start with a delay of about 250 microseconds and vary thatwhile setting rivets in a test panel. Inspection of the rivets willquickly reveal the optimum range, and then sectioning of the rivets inthat range will disclose the best time delay for that workpiece and thatrivet.

The voltage set point set into the logic unit 530 is based on the energynecessary to upset the rivet. It varies with the rivet material, lengthand thickness, but a typical voltage set point would be on the order of300 volts. A capacitance of 0.028 farads charged to a voltage of 300volts will deliver 2000-6000 joules of energy in a characteristiccapacitor discharge curve shown in FIG. 36.

When using headed rivets, an electromagnetic rivet system which hasequal current applied to the coils on both sides can dimple a thin panelbecause the force created by the actuator on the headed side is exertedon both the rivet shank and the rivet head in the countersink, andthrough the countersunk rivet head to the panel. The actuator in theelectromagnetic riveter creates a force greater than the force requiredto yield the rivet shank, therefore the resisting force exerted by therivet shank in opposition to the actuator is insufficient to resist theactuator force. The force balance at the countersink produces a netforce that can yield the panel, creating a dimple.

If the charging voltage to the capacitor for the head side actuator isturned down so that the panel is not yielded, then the overall currentpulse becomes shorter in time as well as in amplitude, since for a givenload the wave form characteristic shape is a function of capacitanceonly as shown in FIG. 36.

Dimpling occurs early in the waveform because of the different functionsperformed by the riveters on the two sides of the rivet. On the buckside, there is work done through the driver on both the actuator recoilmass and the rivet, but on the headed-side there is little work done onthe rivet by comparison because the only deformation of the rivet at theheaded side is of the shank under the head, and a slight deformation ofthe head of a Briles rivet. The total work done by the actuators on therecoil masses on both sides is approximately equivalent. Because of thegreater work required to deform the rivet tail on the buck side, thebuck side device needs more energy than the head side.

As shown in FIG. 37, the force exerted by the transducers on the bothheaded side driver and the buck side driver are equal up until the point538 when the rivet tail begins to yield. The force exerted on the buckside driver by its transducer, represented by the line 540, continues toincrease, but the force, represented by the line 542, transmittedthrough the rivet on the panel levels off because the yield point of therivet has been reached and further force can only increase the rate ofrivet deformation. As the buckside driver moves to deform the tail ofthe rivet and form the rivet button, the buck side driver and transducermove away from the coil, thereby decreasing the coupling of the magneticfield between the coil and the transducer and producing less force onthe buck side driver than on the head side driver for a given equalinput current to the coils. This is represented in FIG. 37 by the arrow547 between the diverging lines 540 (force on the buck side driver) and544 (force on the headed side driver.) The movement of the buck sidedriver is stored as energy of momentum, represented by the arrow 546,which can produce force over time, whereas the headside driver movesonly a (relatively) small amount, and therefore has minimal momentum tobe dissipated over time. That is, the actuator repulsive force pulse isnot extended over time by momentum reduced to work.

The force 542 exerted by the buck side driver through the rivet tail onthe buck side of the panel, limited by the yield strength of the rivettail, is less than the force 544 exerted by the headed side driver onthe headed side of the panel directly through the rivet head in thecountersink. This results in a force differential, represented by thearrow 548. If the force differential 548 between the force 542 on oneside and the force 544 on the other side of the panel is greater thanthe force, arbitrarily illustrated as the arrow 550, needed to yield thepanel, a dimple will result.

As the rivet tail is deformed, the rivet button grows and fills therivet tail die. The greater area of rivet button will permit a transferof more force by the buck side driver on the buck side of the panelthrough the rivet button, so that force 542 begins to increase, asrepresented by the greater slope of the line 542 at 552. The forceexerted by the driver through the rivet button now increases sharply,and actually surpasses (at 554) the force exerted by the transducer onthe buck side driver, because of the momentum energy which was stored inthe moving buck side driver being transformed into work (represented bythe arrow 558) as the driver slows down. Now a reversed differentialforce is exerted on the two sides of the panel, as represented by thearrow 556 and, if greater than the yield strength 550 of the panel, willcreate a reverse dimple, or pimple.

The slope at the beginning of the current/time waveform doesn't changemuch for small changes in voltage settings, with the load and thecapacitance held constant. If the head side voltage is turned downsufficiently so that the panel does not yield, then the buck side forcepulse is largely unopposed later in the waveform by the energy stored asmomentum in the buck side driver which can push the rivet head out ofthe countersink and cause a gap under the rivet head, or cause a pimple.It is actually possible to dimple the panel at the beginning of thestroke, and then create a gap under the head at the end of the stroke.

By delaying the onset of the head side current, the force applied to thehead side is also delayed, the intent being to allow the buck side forceto have increased sufficiently to have partially deformed the tail ofthe rivet (by compressing it against the mass of the headside actuator)and developed sufficient momentum which could exert sufficient force onthe partially formed button at the rivet tail to "back-up" the panel andprevent dimpling by the headside actuator when it fires. The head sidevoltage (hence current and force) would typically be lower as well,which shortens the pulse, but since the pulse on the head side wasdelayed, the forces later in the pulse are essentially balanced on eachside.

A fine balance is required when using the delay method of preventingdimpling. Too much delay could cause a gap under the head if thefollowing head-side pulse doesn't re-seat the head when the head sideactuator fires. Too little delay would still allow a dimple. Too littlehead side voltage will prevent an inward dimple, but might produce anoutward one, as well as a gap under the rivet head if the buck sideforce is unopposed in the later part of its force pulse. If all isbalanced properly, a combination of delay and lower head side voltagewill produce good results.

In the interest of expanding the "sweet spot" of the machine settingsthat produce a riveted panel with no dimple, no head-gap, and the properbutton diameter, it was realized that a higher capacitance on the headedside than on the buck side would reduce the slope of the current pulse,hence force pulse, to the driver on the headed side. This differentialcapacitance provides an additional variable parameter that will permitthe force on the headed side to be reduced to close to that exerted bythe buck side driver through the yielding rivet tail so that the panelwill not be dimpled. This desirable result is accomplished by using acapacitor in the head side actuator power supply that is of highercapacitance than the buck side capacitor, and by setting the buck sidevoltage higher than the head side, since equal voltages would produce amuch longer force pulse on the head side because of the highercapacitance on the head side. It is also necessary to account for thereduced coupling efficiency of the buck side (due to driver movement) atthe start of the rivet deformation phase and for the dissipation ofmomentum of the buck side driver, later in the cycle.

The capacitance C2 in the buck side power supply needs to besubstantially lower (at least about 25% lower) than the head sidecapacitance C1 to achieve a significantly higher current pulse slope onthe buck side, as shown in FIG. 38, which is necessary to preventdimpling, since the buck side needs a higher current because it producesdriver momentum as well as force to yield the rivet, and it has lowercoupling efficiently.

The buck side voltage must be substantially higher than the head side toproduce the much higher peak current required on the tail side. Thehigher voltage, combined with the lower capacitance extends the forcepulse so that it approximately balances the force pulse of the headside.

As shown in FIG. 38, the lower capacitance C2 of the capacitor in thebuck side power supply produces a steeper slope on the capacitor currentdischarge curve and the force/time slope, so a small reverse forcedifferential 560 on the drivers develops early in the cycle, but it isless than the force 550 needed to dimple the panel. When the tail of therivet begins to yield and cause the flattening of the force curve 542'representing the force exerted by the buck side driver through theyielding rivet tail on the buck side of the panel, the forcedifferential 548' is significantly less than the corresponding forcedifferential 548 when the capacitance and voltage were equal, asrepresented in FIG. 37. This reduces the possibility of dimpling thepanel.

The greater capacitance C1 and lower voltage on the headed side resultsin a lower peak on the force curve which corresponds more closely to thepeak force exerted by the buck side driver in discharging its momentuminto the rivet and therethrough to the panel. The reverse differentialforce 556' illustrated in FIG. 38 is significantly less than thecorresponding force differential 556 when the capacitance and voltageare equal, as represented in FIG. 37. This reduces the possibility of apimple on the headed side of the panel.

Turning finally to FIG. 39, a second embodiment of an electromagneticriveter is shown in an arrangement which is identical to the embodimentshown in FIG. 13, except for the electromagnetic actuator and the cableconnection, which are different. The other elements of the device arethe same and have been given the same reference numerals. Thedescription of these same elements given in connection with FIG. 13 alsoapplies to this embodiment of FIG. 39.

A transducer 218' in the form of a heavy copper plate identical to thetransducer in FIG. 13 is fastened in the recess 212 in the recoil mass214 by a screw 562. The transducer shroud 273' is positioned between thetransducer 218' and the recoil mass 214. An annular coil shroud 278' isfastened to the transducer shroud 273' and depends therefrom to coverthe gap between the transducer 218' and the coil 210' when the device isin operation.

The coil 210' is potted in a potting compound 213' and is fastened to aninsulating support ring 564 of epoxy impregnated fiberglass by screws(not shown.) A flexible lead 566 of berillium copper, for example, isconnected in a notch in the support ring 564 by a screw 568 to athreaded insert 570 embedded in the potting compound 213'. The innerturn of the coil 210' is bent down and under the coil at 572 and isclamped between the inner end of the flexible lead 566 and the threadedinsert 570 to provide a hard electrical contact between the coil and thelead 566.

The lead 566 terminates in a vertically disposed section 574 which isfastened by a screw 576 to an insulating terminal block 578. The screw576 also secures a conductor 580 of a power cable 205' to the terminalblock to establish a hard electrical connection between the power cableconductor 580 and the flexible lead 566.

The power cable 205' is connected by a conventional cable connector 582to a stationary cable terminal box 584 which is clamped to one of thecage cylinders 122. An insulating cover 586 is secured to the top 588 ofthe terminal box 586 and the terminal block 578 to guard againstaccidental contact with the conductor 580. The flexible lead 566 iscovered with an insulating coating except for its ends where it mustmake electrical contact with the coil turn 572 and the screw 576.

In operation of the embodiment shown in FIG. 39, after the rivet isinstalled and ready to be upset, the pop-back cylinder 230 isdepressurized and the electromagnetic riveter is pulled down by therecoil cylinders 246 (not shown in FIG. 39.). The lower cylindricalportion 238 of the driver 203 is pulled down against the rivet die 440,compressing the 448 spring and pushing the nose 450 of the die 440 intocontact with the head of the rivet. The vertical travel of the coil 210'causes the flexible lead 566 to flex down and outward approximately tothe position shown at 590 in the dotted lines, resulting in very littleconcentration of the bending stresses at any particular spot in theflexible lead, so the lead life will not be p rematurely shortened byfatigue failure.

The electromagnetic actuator receives a current pulse from the powercable 205' at the stationary terminal box 584 and conducts the currentthrough the flexible lead 566 to the coil 210'. The current flowsthrough the coil and out through a similar flexible lead (not shown) tothe neutral conductor in the power cable.

The current pulse in the coil generates a rapidly increasing magneticfield which is coupled to the abutting transducer. The rising magneticfield in the transducer generates eddy currents which in turn generate amagnetic field in opposition to the coil field. The opposing magneticfields in the coil 210' and the transducer 218' result in a strongrepulsive force between the coil 210' and the transducer 218'. The forceon the coil is exerted through the coil 210', the support ring 564, thedriver 203 and the die 440 to the rivet, all of which are in contact inseries, so the force on the coil is transferred through a hardmechanical path to the rivet. The rivet head deforms very little so thecoil moves less that about 1/8 inch during the power stroke. This motionis easily accommodated by the flexing of the flexible lead 566.

The upper half of the electromagnetic actuator, including the transducer218' the recoil mass 214 and the support plate 216 are repelledupwardly, guided by the key 211 running in the keyway 207 in the quill174. The recoiling actuator compresses the gas in the recoil cylinder246 (shown in FIG. 13) to store the recoil energy which, at the end ofthe recoil stroke, is released to return the actuator to its originalposition, at a gradual speed governed by the damping cylinder 262.

The short stroke of the coil in the embodiment shown in FIG. 39 enablesthe use of the flexible lead 566 between the coil 210' and thestationary terminal box 584. It is of considerable advantage to be ableto connect the power cable 205' to a stationary terminal box because thecable need not be made to withstand the whipping motion it experienceswhen attached to the coil 210 of the embodiment of FIG. 13, whichrecoils for the full length of the recoil stroke. Since the recoilstroke is much longer than the power stroke, and since the motion of therecoiling components is sudden and violent, the power cable 205 in theembodiment of FIG. 13 must be reinforced and strengthened at itsconnection to the coil to give it the service life that a productionmachine designed for continuous service over many years in a factorymust have. This is not a concern in the embodiment of FIG. 39 and thecable 205' can be an ordinary high power cable attached to the terminalbox 584 with a conventional connector 582.

Obviously, numerous modifications and variations of the preferredembodiment disclosed herein will occur to those skilled in the art.Accordingly, it is expressly to be understood that these modificationsand variations, and the equivalents thereof, may be practiced whileremaining within the spirit and scope of this invention, as defined inthe following claims:

I claim:
 1. A method of recovering an undeformed, defective, headedrivet from a countersunk hole in one face of a workpiece,comprising:holding said defective rivet in said hole in said workpieceby pulling a vacuum in an enclosure around a tail end of said rivetopposite said rivet head; moving a device containing a cavity over saiddefective rivet in said hole, with said cavity aligned over saiddefective rivet; pushing said tail end of said defective rivet up from aside of said workpiece opposite to said one face to lift said rivet headout of said countersunk hole and into said cavity; blocking said hole insaid workpiece from said opposite side thereof to release the hold ofsaid vacuum on said defective rivet; suctioning said cavity through asuction port to draw said rivet completely out of said hole and out ofsaid cavity through said suction port.
 2. The method defined in claim 1,further comprising:comparing the length of said rivet against a set ofpredetermined length limits constituting a specified length andallowable length tolerance by holding said rivet head in saidcountersunk hole, while translating a probe to contact said tail end ofsaid rivet with said probe and sensing whether said translation of saidprobe falls within a set of predetermined limits to determine whethersaid rivet is within said length tolerance.
 3. A method as defined inclaim 2, wherein:said sensing step includes moving a vane operativelyattached to said probe along a path on which two spaced sensors arepositioned, spaced apart in the direction of said movement of said vaneby a distance corresponding to the difference between said lengthtolerance; whereby said vane will be sensed by one sensor but not bothsensors if said rivet is within said length limits.
 4. A method asdefined in claim 3, wherein:said probe is a rivet tail die in a rivetingmachine.
 5. A method of removing an undeformed, missized rivet, having afirst end and a second end, from a rivet hole in a workpiece,comprising:determining whether said rivet is an incorrect size for saidrivet hole in said workpiece and, if said rivet is an incorrect size,performing the following steps: holding said rivet in said hole againsta stop, so that said first end is flush with a first surface of saidworkpiece, by pulling a vacuum around said second end of said rivet;moving a suction cavity over said hole and adjacent said first surfaceof said workpiece; applying suction to said suction cavity and releasingsaid hold on said second end of said rivet; and suctioning said missizedrivet out of said hole into said suction cavity.
 6. A method as definedin claim 5, wherein:said determining step includes comparing the lengthof said rivet against a predetermined set of length tolerance limits todetermine if said rivet is within said length tolerance limits,
 7. Amethod as defined in claim 5, further comprising:pushing said rivet witha probe, from said second end of said rivet, out of said hole and intosaid cavity.
 8. A method as defined in claim 7, wherein:said releasingstep includes blocking said hole in said workpiece from a second side,opposite said first side, with said probe to release said hold of saidvacuum on said second end of said rivet.
 9. A method as defined in claim7, wherein said determining step includes:comparing the length of saidrivet against a predetermined set of length tolerance limits todetermine if said rivet is within said length tolerance limits, saidcomparing step includes holding one end of said rivet in a referenceposition and translating said probe into contact with said second end ofsaid rivet, and measuring the translation of said probe from a referenceposition to said contact with said second end of said rivet.
 10. Amethod as defined in claim 5, wherein:said suction cavity is a drillcavity in a shuttle, said drill cavity having a suction port with afitting for attachment of a vacuum hose for suctioning drill chips andmissized rivet out of said drill cavity.